Leverage user_contributed_projects to find recent events.

app/finders/user_recent_events_finder.rb

@@ -12,6 +12,8 @@ class UserRecentEventsFinder
 
   attr_reader :current_user, :target_user, :params
 
+  LIMIT = 20
+
   def initialize(current_user, target_user, params = {})
     @current_user = current_user
     @target_user = target_user
@@ -19,15 +21,44 @@ class UserRecentEventsFinder
   end
 
   def execute
-    target_user
-      .recent_events
-      .merge(projects_for_current_user)
-      .references(:project)
+    recent_events(params[:offset] || 0)
+      .joins(:project)
       .with_associations
-      .limit_recent(20, params[:offset])
+      .limit_recent(LIMIT, params[:offset])
+  end
+
+  private
+
+  def recent_events(offset)
+    sql = <<~SQL
+      (#{projects}) AS projects_for_join
+      JOIN (#{target_events.to_sql}) AS #{Event.table_name}
+        ON #{Event.table_name}.project_id = projects_for_join.id
+    SQL
+
+    # Workaround for https://github.com/rails/rails/issues/24193
+    Event.from([Arel.sql(sql)])
   end
 
-  def projects_for_current_user
-    ProjectsFinder.new(current_user: current_user).execute
+  def target_events
+    Event.where(author: target_user)
+  end
+
+  def projects
+    # Compile a list of projects `current_user` interacted with
+    # and `target_user` is allowed to see.
+
+    authorized = target_user
+      .project_interactions
+      .joins(:project_authorizations)
+      .where(project_authorizations: { user: current_user })
+      .select(:id)
+
+    visible = target_user
+      .project_interactions
+      .where(visibility_level: [Gitlab::VisibilityLevel::INTERNAL, Gitlab::VisibilityLevel::PUBLIC])
+      .select(:id)
+
+    Gitlab::SQL::Union.new([authorized, visible]).to_sql
   end
 end

app/models/user.rb

@@ -114,13 +114,15 @@ class User < ActiveRecord::Base
   has_many :project_authorizations
   has_many :authorized_projects, through: :project_authorizations, source: :project
 
+  has_many :user_interacted_projects
+  has_many :project_interactions, through: :user_interacted_projects, source: :project, class_name: 'Project'
+
   has_many :snippets,                 dependent: :destroy, foreign_key: :author_id # rubocop:disable Cop/ActiveRecordDependent
   has_many :notes,                    dependent: :destroy, foreign_key: :author_id # rubocop:disable Cop/ActiveRecordDependent
   has_many :issues,                   dependent: :destroy, foreign_key: :author_id # rubocop:disable Cop/ActiveRecordDependent
   has_many :merge_requests,           dependent: :destroy, foreign_key: :author_id # rubocop:disable Cop/ActiveRecordDependent
   has_many :events,                   dependent: :destroy, foreign_key: :author_id # rubocop:disable Cop/ActiveRecordDependent
   has_many :subscriptions,            dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
-  has_many :recent_events, -> { order "id DESC" }, foreign_key: :author_id,   class_name: "Event"
   has_many :oauth_applications, class_name: 'Doorkeeper::Application', as: :owner, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
   has_one  :abuse_report,             dependent: :destroy, foreign_key: :user_id # rubocop:disable Cop/ActiveRecordDependent
   has_many :reported_abuse_reports,   dependent: :destroy, foreign_key: :reporter_id, class_name: "AbuseReport" # rubocop:disable Cop/ActiveRecordDependent

changelogs/unreleased/40525-listing-user-activity-timeouts.yml

+---
+title: Improve database response time for user activity listing.
+merge_request: 17454
+author:
+type: performance

spec/models/user_spec.rb

@@ -27,7 +27,6 @@ describe User do
     it { is_expected.to have_many(:keys).dependent(:destroy) }
     it { is_expected.to have_many(:deploy_keys).dependent(:destroy) }
     it { is_expected.to have_many(:events).dependent(:destroy) }
-    it { is_expected.to have_many(:recent_events).class_name('Event') }
     it { is_expected.to have_many(:issues).dependent(:destroy) }
     it { is_expected.to have_many(:notes).dependent(:destroy) }
     it { is_expected.to have_many(:merge_requests).dependent(:destroy) }