Increase poll interval for merged MRs widget

app/controllers/projects/merge_requests/content_controller.rb

@@ -10,6 +10,9 @@ class Projects::MergeRequests::ContentController < Projects::MergeRequests::Appl
   before_action :set_polling_header
   around_action :allow_gitaly_ref_name_caching
 
+  FAST_POLLING_INTERVAL = 10.seconds.in_milliseconds
+  SLOW_POLLING_INTERVAL = 5.minutes.in_milliseconds
+
   def widget
     respond_to do |format|
       format.json do
@@ -29,7 +32,8 @@ class Projects::MergeRequests::ContentController < Projects::MergeRequests::Appl
   private
 
   def set_polling_header
-    Gitlab::PollingInterval.set_header(response, interval: 10_000)
+    interval = merge_request.open? ? FAST_POLLING_INTERVAL : SLOW_POLLING_INTERVAL
+    Gitlab::PollingInterval.set_header(response, interval: interval)
   end
 
   def serializer(entity)

changelogs/unreleased/id-increase-poll-interval-for-merged-mrs.yml

+---
+title: Increase poll interval for merged MRs widget
+merge_request: 39961
+author:
+type: performance

spec/controllers/projects/merge_requests/content_controller_spec.rb

@@ -48,6 +48,8 @@ RSpec.describe Projects::MergeRequests::ContentController do
         expect(merge_request).to receive(:check_mergeability)
 
         do_request(:widget)
+
+        expect(response.headers['Poll-Interval']).to eq('10000')
       end
 
       context 'merged merge request' do
@@ -59,6 +61,7 @@ RSpec.describe Projects::MergeRequests::ContentController do
           do_request(:widget)
 
           expect(response).to match_response_schema('entities/merge_request_poll_widget')
+          expect(response.headers['Poll-Interval']).to eq('300000')
         end
       end
     end