diff --git a/app/services/search/global_service.rb b/app/services/search/global_service.rb
index ff188102b62..92a32e703af 100644
--- a/app/services/search/global_service.rb
+++ b/app/services/search/global_service.rb
@@ -1,13 +1,16 @@
 module Search
   class GlobalService
     attr_accessor :current_user, :params
+    attr_reader :default_project_filter
 
     def initialize(user, params)
       @current_user, @params = user, params.dup
+      @default_project_filter = true
     end
 
     def execute
-      Gitlab::SearchResults.new(current_user, projects, params[:search])
+      Gitlab::SearchResults.new(current_user, projects, params[:search],
+                                default_project_filter: default_project_filter)
     end
 
     def projects
diff --git a/app/services/search/group_service.rb b/app/services/search/group_service.rb
index 29478e3251f..b4efba68715 100644
--- a/app/services/search/group_service.rb
+++ b/app/services/search/group_service.rb
@@ -5,6 +5,7 @@ module Search
     def initialize(user, group, params)
       super(user, params)
 
+      @default_project_filter = false
       @group = group
     end
 
diff --git a/changelogs/unreleased/15955-improve-search-query.yml b/changelogs/unreleased/15955-improve-search-query.yml
new file mode 100644
index 00000000000..80cb8af617f
--- /dev/null
+++ b/changelogs/unreleased/15955-improve-search-query.yml
@@ -0,0 +1,5 @@
+---
+title: Improve search query for merge requests.
+merge_request:
+author:
+type: performance
diff --git a/lib/gitlab/search_results.rb b/lib/gitlab/search_results.rb
index fef9d3e31d4..7037e2e61cc 100644
--- a/lib/gitlab/search_results.rb
+++ b/lib/gitlab/search_results.rb
@@ -27,10 +27,17 @@ module Gitlab
     # It allows us to search only for projects user has access to
     attr_reader :limit_projects
 
-    def initialize(current_user, limit_projects, query)
+    # Whether a custom filter is used to restrict scope of projects.
+    # If the default filter (which lists all projects user has access to)
+    # is used, we can skip it when filtering merge requests and optimize the
+    # query
+    attr_reader :default_project_filter
+
+    def initialize(current_user, limit_projects, query, default_project_filter: false)
       @current_user = current_user
       @limit_projects = limit_projects || Project.all
       @query = query
+      @default_project_filter = default_project_filter
     end
 
     def objects(scope, page = nil)
@@ -94,7 +101,11 @@ module Gitlab
     end
 
     def merge_requests
-      merge_requests = MergeRequestsFinder.new(current_user).execute.in_projects(project_ids_relation)
+      merge_requests = MergeRequestsFinder.new(current_user).execute
+      unless default_project_filter
+        merge_requests = merge_requests.in_projects(project_ids_relation)
+      end
+
       merge_requests =
         if query =~ /[#!](\d+)\z/
           merge_requests.where(iid: $1)
diff --git a/spec/lib/gitlab/search_results_spec.rb b/spec/lib/gitlab/search_results_spec.rb
index e44a7c23452..a958baab44f 100644
--- a/spec/lib/gitlab/search_results_spec.rb
+++ b/spec/lib/gitlab/search_results_spec.rb
@@ -51,6 +51,17 @@ describe Gitlab::SearchResults do
 
       expect(results.objects('merge_requests')).to include merge_request_2
     end
+
+    it 'includes project filter by default' do
+      expect(results).to receive(:project_ids_relation).and_call_original
+      results.objects('merge_requests')
+    end
+
+    it 'it skips project filter if default is used' do
+      allow(results).to receive(:default_project_filter).and_return(true)
+      expect(results).not_to receive(:project_ids_relation)
+      results.objects('merge_requests')
+    end
   end
 
   it 'does not list issues on private projects' do