Optimize count query

Only count up to the point we're interested in

ee/changelogs/unreleased/ab-60524-optimize-query.yml

+---
+title: Limit count to improve query performance
+merge_request: 12231
+author:
+type: performance

ee/lib/gitlab/mirror.rb

@@ -37,11 +37,11 @@ module Gitlab
         return false if available_spots < capacity_threshold
 
         # Only reschedule if we are able to completely fill up the available spots.
-        mirrors_ready_to_sync_count >= available_spots
+        mirrors_ready_to_sync_count(available_spots + 1) >= available_spots
       end
 
-      def mirrors_ready_to_sync_count
-        Project.mirrors_to_sync(Time.now).count
+      def mirrors_ready_to_sync_count(up_to = nil)
+        Project.mirrors_to_sync(Time.now).limit(up_to).count
       end
 
       def available_capacity