diff --git a/app/controllers/concerns/renders_commits.rb b/app/controllers/concerns/renders_commits.rb
index fb41dc1e8a8..b1c9b1e532f 100644
--- a/app/controllers/concerns/renders_commits.rb
+++ b/app/controllers/concerns/renders_commits.rb
@@ -1,4 +1,24 @@
 module RendersCommits
+  def limited_commits(commits)
+    if commits.size > MergeRequestDiff::COMMITS_SAFE_SIZE
+      [
+        commits.first(MergeRequestDiff::COMMITS_SAFE_SIZE),
+        commits.size - MergeRequestDiff::COMMITS_SAFE_SIZE
+      ]
+    else
+      [commits, 0]
+    end
+  end
+
+  # This is used as a helper method in a controller.
+  # rubocop: disable Gitlab/ModuleWithInstanceVariables
+  def set_commits_for_rendering(commits)
+    @total_commit_count = commits.size
+    limited, @hidden_commit_count = limited_commits(commits)
+    prepare_commits_for_rendering(limited)
+  end
+  # rubocop: enable Gitlab/ModuleWithInstanceVariables
+
   def prepare_commits_for_rendering(commits)
     Banzai::CommitRenderer.render(commits, @project, current_user) # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
diff --git a/app/controllers/projects/commits_controller.rb b/app/controllers/projects/commits_controller.rb
index 36faea8056e..5546bef850b 100644
--- a/app/controllers/projects/commits_controller.rb
+++ b/app/controllers/projects/commits_controller.rb
@@ -63,7 +63,7 @@ class Projects::CommitsController < Projects::ApplicationController
       end
 
     @commits = @commits.with_pipeline_status
-    @commits = prepare_commits_for_rendering(@commits)
+    @commits = set_commits_for_rendering(@commits)
   end
 
   # Rails 5 sets request.format from the extension.
diff --git a/app/controllers/projects/compare_controller.rb b/app/controllers/projects/compare_controller.rb
index f93e500a07a..a1e12821caf 100644
--- a/app/controllers/projects/compare_controller.rb
+++ b/app/controllers/projects/compare_controller.rb
@@ -78,7 +78,7 @@ class Projects::CompareController < Projects::ApplicationController
   end
 
   def define_commits
-    @commits = compare.present? ? prepare_commits_for_rendering(compare.commits) : []
+    @commits = compare.present? ? set_commits_for_rendering(@compare.commits) : []
   end
 
   def define_diffs
diff --git a/app/controllers/projects/merge_requests/creations_controller.rb b/app/controllers/projects/merge_requests/creations_controller.rb
index 81129456ad8..03d0290ac1d 100644
--- a/app/controllers/projects/merge_requests/creations_controller.rb
+++ b/app/controllers/projects/merge_requests/creations_controller.rb
@@ -101,7 +101,7 @@ class Projects::MergeRequests::CreationsController < Projects::MergeRequests::Ap
 
     @target_project = @merge_request.target_project
     @source_project = @merge_request.source_project
-    @commits = prepare_commits_for_rendering(@merge_request.commits)
+    @commits = set_commits_for_rendering(@merge_request.commits)
     @commit = @merge_request.diff_head_commit
 
     @labels = LabelsFinder.new(current_user, project_id: @project.id).execute
diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 1b069fe507b..d31b58972ca 100644
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/app/controllers/projects/merge_requests_controller.rb
@@ -79,7 +79,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     # Get commits from repository
     # or from cache if already merged
     @commits =
-      prepare_commits_for_rendering(@merge_request.commits.with_pipeline_status)
+      set_commits_for_rendering(@merge_request.commits.with_pipeline_status)
 
     render json: { html: view_to_html_string('projects/merge_requests/_commits') }
   end
diff --git a/app/helpers/commits_helper.rb b/app/helpers/commits_helper.rb
index 89fe90fd801..7a942c44ac4 100644
--- a/app/helpers/commits_helper.rb
+++ b/app/helpers/commits_helper.rb
@@ -210,17 +210,6 @@ module CommitsHelper
     Sanitize.clean(string, remove_contents: true)
   end
 
-  def limited_commits(commits)
-    if commits.size > MergeRequestDiff::COMMITS_SAFE_SIZE
-      [
-        commits.first(MergeRequestDiff::COMMITS_SAFE_SIZE),
-        commits.size - MergeRequestDiff::COMMITS_SAFE_SIZE
-      ]
-    else
-      [commits, 0]
-    end
-  end
-
   def commit_path(project, commit, merge_request: nil)
     if merge_request&.persisted?
       diffs_project_merge_request_path(project, merge_request, commit_id: commit.id)
diff --git a/app/views/projects/commits/_commit_list.html.haml b/app/views/projects/commits/_commit_list.html.haml
index 8f8eb2c3d5a..6ed65d07202 100644
--- a/app/views/projects/commits/_commit_list.html.haml
+++ b/app/views/projects/commits/_commit_list.html.haml
@@ -1,9 +1,10 @@
-- commits, hidden = limited_commits(@commits)
+- commits = @commits
+- hidden = @hidden_commit_count
 - commits = Commit.decorate(commits, @project)
 
 .card
   .card-header
-    Commits (#{@commits.count})
+    Commits (#{@total_commit_count})
   - if hidden > 0
     %ul.content-list
       - commits.each do |commit|
diff --git a/app/views/projects/commits/_commits.html.haml b/app/views/projects/commits/_commits.html.haml
index ac6852751be..ec05ff50f25 100644
--- a/app/views/projects/commits/_commits.html.haml
+++ b/app/views/projects/commits/_commits.html.haml
@@ -2,7 +2,8 @@
 - project = local_assigns.fetch(:project) { merge_request&.project }
 - ref = local_assigns.fetch(:ref) { merge_request&.source_branch }
 
-- commits, hidden = limited_commits(@commits)
+- commits = @commits
+- hidden = @hidden_commit_count
 
 - commits.chunk { |c| c.committed_date.in_time_zone.to_date }.each do |day, commits|
   %li.commit-header.js-commit-header{ data: { day: day } }
diff --git a/app/views/projects/merge_requests/creations/_new_submit.html.haml b/app/views/projects/merge_requests/creations/_new_submit.html.haml
index f7a5d85500f..d5c4134dee2 100644
--- a/app/views/projects/merge_requests/creations/_new_submit.html.haml
+++ b/app/views/projects/merge_requests/creations/_new_submit.html.haml
@@ -33,7 +33,7 @@
             %li.commits-tab.new-tab
               = link_to url_for(safe_params), data: {target: 'div#commits', action: 'new', toggle: 'tabvue'} do
                 Commits
-                %span.badge.badge-pill= @commits.size
+                %span.badge.badge-pill= @total_commit_count
             - if @pipelines.any?
               %li.builds-tab
                 = link_to url_for(safe_params.merge(action: 'pipelines')), data: {target: 'div#pipelines', action: 'pipelines', toggle: 'tabvue'} do
diff --git a/changelogs/unreleased/sh-limit-commit-renderering.yml b/changelogs/unreleased/sh-limit-commit-renderering.yml
new file mode 100644
index 00000000000..c44c67bcc90
--- /dev/null
+++ b/changelogs/unreleased/sh-limit-commit-renderering.yml
@@ -0,0 +1,5 @@
+---
+title: Speed up diff comparisons by limiting number of commit messages rendered
+merge_request: 21335
+author:
+type: performance
diff --git a/spec/controllers/projects/merge_requests/creations_controller_spec.rb b/spec/controllers/projects/merge_requests/creations_controller_spec.rb
index d8995f98575..f8c37c0a676 100644
--- a/spec/controllers/projects/merge_requests/creations_controller_spec.rb
+++ b/spec/controllers/projects/merge_requests/creations_controller_spec.rb
@@ -29,6 +29,55 @@ describe Projects::MergeRequests::CreationsController do
         expect(response).to be_success
       end
     end
+
+    context 'merge request with some commits' do
+      render_views
+
+      let(:large_diff_params) do
+        {
+          namespace_id: fork_project.namespace.to_param,
+          project_id: fork_project,
+          merge_request: {
+            source_branch: 'master',
+            target_branch: 'fix'
+          }
+        }
+      end
+
+      describe 'with artificial limits' do
+        before do
+          # Load MergeRequestdiff so stub_const won't override it with its own definition
+          # See https://github.com/rspec/rspec-mocks/issues/1079
+          stub_const("#{MergeRequestDiff}::COMMITS_SAFE_SIZE", 2)
+        end
+
+        it 'limits total commits' do
+          get :new, large_diff_params
+
+          expect(response).to be_success
+
+          total = assigns(:total_commit_count)
+          expect(assigns(:commits)).to be_an Array
+          expect(total).to be > 0
+          expect(assigns(:hidden_commit_count)).to be > 0
+          expect(response).to have_gitlab_http_status(200)
+          expect(response.body).to match %r(<span class="commits-count">2 commits</span>)
+        end
+      end
+
+      it 'shows total commits' do
+        get :new, large_diff_params
+
+        expect(response).to be_success
+
+        total = assigns(:total_commit_count)
+        expect(assigns(:commits)).to be_an Array
+        expect(total).to be > 0
+        expect(assigns(:hidden_commit_count)).to eq(0)
+        expect(response).to have_gitlab_http_status(200)
+        expect(response.body).to match %r(<span class="commits-count">#{total} commits</span>)
+      end
+    end
   end
 
   describe 'GET diffs' do
diff --git a/spec/views/projects/merge_requests/_commits.html.haml_spec.rb b/spec/views/projects/merge_requests/_commits.html.haml_spec.rb
index b1c6565c08a..a7628548de6 100644
--- a/spec/views/projects/merge_requests/_commits.html.haml_spec.rb
+++ b/spec/views/projects/merge_requests/_commits.html.haml_spec.rb
@@ -20,6 +20,7 @@ describe 'projects/merge_requests/_commits.html.haml' do
 
     assign(:merge_request, merge_request)
     assign(:commits, merge_request.commits)
+    assign(:hidden_commit_count, 0)
   end
 
   it 'shows commits from source project' do
@@ -30,4 +31,16 @@ describe 'projects/merge_requests/_commits.html.haml' do
 
     expect(rendered).to have_link(href: href)
   end
+
+  context 'when there are hidden commits' do
+    before do
+      assign(:hidden_commit_count, 1)
+    end
+
+    it 'shows notice about omitted commits' do
+      render
+
+      expect(rendered).to match(/1 additional commit has been omitted to prevent performance issues/)
+    end
+  end
 end
diff --git a/spec/views/projects/merge_requests/creations/_new_submit.html.haml_spec.rb b/spec/views/projects/merge_requests/creations/_new_submit.html.haml_spec.rb
index 9ab105c3238..8befae39d3a 100644
--- a/spec/views/projects/merge_requests/creations/_new_submit.html.haml_spec.rb
+++ b/spec/views/projects/merge_requests/creations/_new_submit.html.haml_spec.rb
@@ -9,6 +9,8 @@ describe 'projects/merge_requests/creations/_new_submit.html.haml' do
 
     assign(:merge_request, merge_request)
     assign(:commits, merge_request.commits)
+    assign(:hidden_commit_count, 0)
+    assign(:total_commit_count, merge_request.commits.count)
     assign(:project, merge_request.target_project)
 
     allow(view).to receive(:can?).and_return(true)
@@ -29,4 +31,17 @@ describe 'projects/merge_requests/creations/_new_submit.html.haml' do
       expect(rendered).not_to have_text('Builds')
     end
   end
+
+  context 'when there are hidden commits' do
+    before do
+      assign(:pipelines, Ci::Pipeline.none)
+      assign(:hidden_commit_count, 2)
+    end
+
+    it 'shows notice about omitted commits' do
+      render
+
+      expect(rendered).to match(/2 additional commits have been omitted to prevent performance issues/)
+    end
+  end
 end