diff --git a/app/assets/javascripts/issuable_context.js b/app/assets/javascripts/issuable_context.js
index 70c364e51fe..1d305f1eb2f 100644
--- a/app/assets/javascripts/issuable_context.js
+++ b/app/assets/javascripts/issuable_context.js
@@ -67,10 +67,13 @@ const PARTICIPANTS_ROW_COUNT = 7;
       originalText = $(this).data("original-text");
       if (currentText === originalText) {
         $(this).text(lessText);
+
+        if (gl.lazyLoader) gl.lazyLoader.loadCheck();
       } else {
         $(this).text(originalText);
       }
-      return $(".js-participants-hidden").toggle();
+
+      $(".js-participants-hidden").toggle();
     };
 
     return IssuableContext;
diff --git a/app/assets/javascripts/right_sidebar.js b/app/assets/javascripts/right_sidebar.js
index 0c1ec276baf..a41548bd694 100644
--- a/app/assets/javascripts/right_sidebar.js
+++ b/app/assets/javascripts/right_sidebar.js
@@ -29,30 +29,32 @@ import Cookies from 'js-cookie';
       $('.dropdown').on('loading.gl.dropdown', this.sidebarDropdownLoading);
       $('.dropdown').on('loaded.gl.dropdown', this.sidebarDropdownLoaded);
 
-      $document.on('click', '.js-sidebar-toggle', function(e, triggered) {
-        var $allGutterToggleIcons, $this, $thisIcon;
-        e.preventDefault();
-        $this = $(this);
-        $thisIcon = $this.find('i');
-        $allGutterToggleIcons = $('.js-sidebar-toggle i');
-        if ($thisIcon.hasClass('fa-angle-double-right')) {
-          $allGutterToggleIcons.removeClass('fa-angle-double-right').addClass('fa-angle-double-left');
-          $('aside.right-sidebar').removeClass('right-sidebar-expanded').addClass('right-sidebar-collapsed');
-          $('.page-with-sidebar').removeClass('right-sidebar-expanded').addClass('right-sidebar-collapsed');
-        } else {
-          $allGutterToggleIcons.removeClass('fa-angle-double-left').addClass('fa-angle-double-right');
-          $('aside.right-sidebar').removeClass('right-sidebar-collapsed').addClass('right-sidebar-expanded');
-          $('.page-with-sidebar').removeClass('right-sidebar-collapsed').addClass('right-sidebar-expanded');
-
-          if (gl.lazyLoader) gl.lazyLoader.loadCheck();
-        }
-        if (!triggered) {
-          return Cookies.set("collapsed_gutter", $('.right-sidebar').hasClass('right-sidebar-collapsed'));
-        }
-      });
+      $document.on('click', '.js-sidebar-toggle', this.sidebarToggleClicked);
       return $(document).off('click', '.js-issuable-todo').on('click', '.js-issuable-todo', this.toggleTodo);
     };
 
+    Sidebar.prototype.sidebarToggleClicked = function (e, triggered) {
+      var $allGutterToggleIcons, $this, $thisIcon;
+      e.preventDefault();
+      $this = $(this);
+      $thisIcon = $this.find('i');
+      $allGutterToggleIcons = $('.js-sidebar-toggle i');
+      if ($thisIcon.hasClass('fa-angle-double-right')) {
+        $allGutterToggleIcons.removeClass('fa-angle-double-right').addClass('fa-angle-double-left');
+        $('aside.right-sidebar').removeClass('right-sidebar-expanded').addClass('right-sidebar-collapsed');
+        $('.page-with-sidebar').removeClass('right-sidebar-expanded').addClass('right-sidebar-collapsed');
+      } else {
+        $allGutterToggleIcons.removeClass('fa-angle-double-left').addClass('fa-angle-double-right');
+        $('aside.right-sidebar').removeClass('right-sidebar-collapsed').addClass('right-sidebar-expanded');
+        $('.page-with-sidebar').removeClass('right-sidebar-collapsed').addClass('right-sidebar-expanded');
+
+        if (gl.lazyLoader) gl.lazyLoader.loadCheck();
+      }
+      if (!triggered) {
+        Cookies.set("collapsed_gutter", $('.right-sidebar').hasClass('right-sidebar-collapsed'));
+      }
+    };
+
     Sidebar.prototype.toggleTodo = function(e) {
       var $btnText, $this, $todoLoading, ajaxType, url;
       $this = $(e.currentTarget);
diff --git a/app/helpers/projects_helper.rb b/app/helpers/projects_helper.rb
index 21fb17e06d6..4c0cce54527 100644
--- a/app/helpers/projects_helper.rb
+++ b/app/helpers/projects_helper.rb
@@ -21,11 +21,14 @@ module ProjectsHelper
     classes = %W[avatar avatar-inline s#{opts[:size]}]
     classes << opts[:avatar_class] if opts[:avatar_class]
 
-    image_tag(avatar_icon(author, opts[:size]), width: opts[:size], class: classes, alt: '')
+    avatar = avatar_icon(author, opts[:size])
+    src = opts[:lazy_load] ? nil : avatar
+
+    image_tag(src, width: opts[:size], class: classes, alt: '', "data-src" => avatar)
   end
 
   def link_to_member(project, author, opts = {}, &block)
-    default_opts = { avatar: true, name: true, size: 16, author_class: 'author', title: ":name", tooltip: false }
+    default_opts = { avatar: true, name: true, size: 16, author_class: 'author', title: ":name", tooltip: false, lazy_load: false }
     opts = default_opts.merge(opts)
 
     return "(deleted)" unless author
diff --git a/app/views/shared/issuable/_participants.html.haml b/app/views/shared/issuable/_participants.html.haml
index 8a71819aa8e..d2b62557e03 100644
--- a/app/views/shared/issuable/_participants.html.haml
+++ b/app/views/shared/issuable/_participants.html.haml
@@ -11,7 +11,7 @@
   .hide-collapsed.participants-list
     - participants.each do |participant|
       .participants-author.js-participants-author
-        = link_to_member(@project, participant, name: false, size: 24)
+        = link_to_member(@project, participant, name: false, size: 24, lazy_load: true)
   - if participants_extra > 0
     .hide-collapsed.participants-more
       %a.js-participants-more{ href: "#", data: { original_text: "+ #{participants_size - 7} more", less_text: "- show less" } }
diff --git a/changelogs/unreleased/34366-issue-sidebar-don-t-render-participants-in-collapsed-state.yml b/changelogs/unreleased/34366-issue-sidebar-don-t-render-participants-in-collapsed-state.yml
new file mode 100644
index 00000000000..d34e685b5f5
--- /dev/null
+++ b/changelogs/unreleased/34366-issue-sidebar-don-t-render-participants-in-collapsed-state.yml
@@ -0,0 +1,5 @@
+---
+title: Load sidebar participants avatars only when visible
+merge_request: 14270
+author:
+type: other
diff --git a/spec/helpers/projects_helper_spec.rb b/spec/helpers/projects_helper_spec.rb
index 7ded95d01af..641971485de 100644
--- a/spec/helpers/projects_helper_spec.rb
+++ b/spec/helpers/projects_helper_spec.rb
@@ -200,13 +200,13 @@ describe ProjectsHelper do
     end
 
     it 'returns image tag for member avatar' do
-      expect(helper).to receive(:image_tag).with(expected, { width: 16, class: ["avatar", "avatar-inline", "s16"], alt: "" })
+      expect(helper).to receive(:image_tag).with(expected, { width: 16, class: ["avatar", "avatar-inline", "s16"], alt: "", "data-src" => anything })
 
       helper.link_to_member_avatar(user)
     end
 
     it 'returns image tag with avatar class' do
-      expect(helper).to receive(:image_tag).with(expected, { width: 16, class: ["avatar", "avatar-inline", "s16", "any-avatar-class"], alt: "" })
+      expect(helper).to receive(:image_tag).with(expected, { width: 16, class: ["avatar", "avatar-inline", "s16", "any-avatar-class"], alt: "", "data-src" => anything })
 
       helper.link_to_member_avatar(user, avatar_class: "any-avatar-class")
     end
diff --git a/spec/javascripts/issuable_context_spec.js b/spec/javascripts/issuable_context_spec.js
new file mode 100644
index 00000000000..bcb2b7b24a0
--- /dev/null
+++ b/spec/javascripts/issuable_context_spec.js
@@ -0,0 +1,34 @@
+/* global IssuableContext */
+import '~/issuable_context';
+import $ from 'jquery';
+
+describe('IssuableContext', () => {
+  describe('toggleHiddenParticipants', () => {
+    const event = jasmine.createSpyObj('event', ['preventDefault']);
+
+    beforeEach(() => {
+      spyOn($.fn, 'data').and.returnValue('data');
+      spyOn($.fn, 'text').and.returnValue('data');
+    });
+
+    afterEach(() => {
+      gl.lazyLoader = undefined;
+    });
+
+    it('calls loadCheck if lazyLoader is set', () => {
+      gl.lazyLoader = jasmine.createSpyObj('lazyLoader', ['loadCheck']);
+
+      IssuableContext.prototype.toggleHiddenParticipants(event);
+
+      expect(gl.lazyLoader.loadCheck).toHaveBeenCalled();
+    });
+
+    it('does not throw if lazyLoader is not defined', () => {
+      gl.lazyLoader = undefined;
+
+      const toggle = IssuableContext.prototype.toggleHiddenParticipants.bind(null, event);
+
+      expect(toggle).not.toThrow();
+    });
+  });
+});
diff --git a/spec/javascripts/right_sidebar_spec.js b/spec/javascripts/right_sidebar_spec.js
index f2072a6f350..5505f983d71 100644
--- a/spec/javascripts/right_sidebar_spec.js
+++ b/spec/javascripts/right_sidebar_spec.js
@@ -32,56 +32,86 @@ import '~/right_sidebar';
   };
 
   describe('RightSidebar', function() {
-    var fixtureName = 'issues/open-issue.html.raw';
-    preloadFixtures(fixtureName);
-    loadJSONFixtures('todos/todos.json');
+    describe('fixture tests', () => {
+      var fixtureName = 'issues/open-issue.html.raw';
+      preloadFixtures(fixtureName);
+      loadJSONFixtures('todos/todos.json');
 
-    beforeEach(function() {
-      loadFixtures(fixtureName);
-      this.sidebar = new Sidebar;
-      $aside = $('.right-sidebar');
-      $page = $('.page-with-sidebar');
-      $icon = $aside.find('i');
-      $toggle = $aside.find('.js-sidebar-toggle');
-      return $labelsIcon = $aside.find('.sidebar-collapsed-icon');
-    });
-    it('should expand/collapse the sidebar when arrow is clicked', function() {
-      assertSidebarState('expanded');
-      $toggle.click();
-      assertSidebarState('collapsed');
-      $toggle.click();
-      assertSidebarState('expanded');
-    });
-    it('should float over the page and when sidebar icons clicked', function() {
-      $labelsIcon.click();
-      return assertSidebarState('expanded');
-    });
-    it('should collapse when the icon arrow clicked while it is floating on page', function() {
-      $labelsIcon.click();
-      assertSidebarState('expanded');
-      $toggle.click();
-      return assertSidebarState('collapsed');
-    });
-
-    it('should broadcast todo:toggle event when add todo clicked', function() {
-      var todos = getJSONFixture('todos/todos.json');
-      spyOn(jQuery, 'ajax').and.callFake(function() {
-        var d = $.Deferred();
-        var response = todos;
-        d.resolve(response);
-        return d.promise();
+      beforeEach(function() {
+        loadFixtures(fixtureName);
+        this.sidebar = new Sidebar;
+        $aside = $('.right-sidebar');
+        $page = $('.page-with-sidebar');
+        $icon = $aside.find('i');
+        $toggle = $aside.find('.js-sidebar-toggle');
+        return $labelsIcon = $aside.find('.sidebar-collapsed-icon');
+      });
+      it('should expand/collapse the sidebar when arrow is clicked', function() {
+        assertSidebarState('expanded');
+        $toggle.click();
+        assertSidebarState('collapsed');
+        $toggle.click();
+        assertSidebarState('expanded');
+      });
+      it('should float over the page and when sidebar icons clicked', function() {
+        $labelsIcon.click();
+        return assertSidebarState('expanded');
+      });
+      it('should collapse when the icon arrow clicked while it is floating on page', function() {
+        $labelsIcon.click();
+        assertSidebarState('expanded');
+        $toggle.click();
+        return assertSidebarState('collapsed');
       });
 
-      var todoToggleSpy = spyOnEvent(document, 'todo:toggle');
+      it('should broadcast todo:toggle event when add todo clicked', function() {
+        var todos = getJSONFixture('todos/todos.json');
+        spyOn(jQuery, 'ajax').and.callFake(function() {
+          var d = $.Deferred();
+          var response = todos;
+          d.resolve(response);
+          return d.promise();
+        });
 
-      $('.issuable-sidebar-header .js-issuable-todo').click();
+        var todoToggleSpy = spyOnEvent(document, 'todo:toggle');
 
-      expect(todoToggleSpy.calls.count()).toEqual(1);
+        $('.issuable-sidebar-header .js-issuable-todo').click();
+
+        expect(todoToggleSpy.calls.count()).toEqual(1);
+      });
+
+      it('should not hide collapsed icons', () => {
+        [].forEach.call(document.querySelectorAll('.sidebar-collapsed-icon'), (el) => {
+          expect(el.querySelector('.fa, svg').classList.contains('hidden')).toBeFalsy();
+        });
+      });
     });
 
-    it('should not hide collapsed icons', () => {
-      [].forEach.call(document.querySelectorAll('.sidebar-collapsed-icon'), (el) => {
-        expect(el.querySelector('.fa, svg').classList.contains('hidden')).toBeFalsy();
+    describe('sidebarToggleClicked', () => {
+      const event = jasmine.createSpyObj('event', ['preventDefault']);
+
+      beforeEach(() => {
+        spyOn($.fn, 'hasClass').and.returnValue(false);
+      });
+
+      afterEach(() => {
+        gl.lazyLoader = undefined;
+      });
+
+      it('calls loadCheck if lazyLoader is set', () => {
+        gl.lazyLoader = jasmine.createSpyObj('lazyLoader', ['loadCheck']);
+
+        Sidebar.prototype.sidebarToggleClicked(event);
+
+        expect(gl.lazyLoader.loadCheck).toHaveBeenCalled();
+      });
+
+      it('does not throw if lazyLoader is not defined', () => {
+        gl.lazyLoader = undefined;
+
+        const toggle = Sidebar.prototype.sidebarToggleClicked.bind(null, event);
+
+        expect(toggle).not.toThrow();
       });
     });
   });
diff --git a/spec/views/shared/issuable/_participants.html.haml.rb b/spec/views/shared/issuable/_participants.html.haml.rb
new file mode 100644
index 00000000000..51059d4c0d7
--- /dev/null
+++ b/spec/views/shared/issuable/_participants.html.haml.rb
@@ -0,0 +1,26 @@
+require 'spec_helper'
+require 'nokogiri'
+
+describe 'shared/issuable/_participants.html.haml' do
+  let(:project) { create(:project) }
+  let(:participants)  { create_list(:user, 100) }
+
+  before do
+    allow(view).to receive_messages(project: project,
+                                    participants: participants)
+  end
+
+  it 'renders lazy loaded avatars' do
+    render 'shared/issuable/participants'
+
+    html = Nokogiri::HTML(rendered)
+
+    avatars = html.css('.participants-author img')
+
+    avatars.each do |avatar|
+      expect(avatar[:class]).to include('lazy')
+      expect(avatar[:src]).to eql(LazyImageTagHelper.placeholder_image)
+      expect(avatar[:"data-src"]).to match('http://www.gravatar.com/avatar/')
+    end
+  end
+end