Reduce number of queries when calling GlobalMilestone#{labels,participants}

CHANGELOG

@@ -180,6 +180,7 @@ v 8.12.0 (unreleased)
   - Clean environment variables when running git hooks
   - Fix Import/Export issues importing protected branches and some specific models
   - Fix non-master branch readme display in tree view
+  - Speed-up group milestones show page
   - Add UX improvements for merge request version diffs
 v 8.11.7

app/models/global_milestone.rb

@@ -61,11 +61,11 @@ class GlobalMilestone
   end
   def participants
-    @participants ||= milestones.map(&:participants).flatten.compact.uniq
+    @participants ||= milestones_relation.includes(:participants).map(&:participants).flatten.compact.uniq
   end
   def labels
-    @labels ||= GlobalLabel.build_collection(milestones.map(&:labels).flatten)
+    @labels ||= GlobalLabel.build_collection(milestones_relation.includes(:labels).map(&:labels).flatten)
                            .sort_by!(&:title)
   end
@@ -89,4 +89,14 @@ class GlobalMilestone
       end
     end
   end
+
+  private
+
+  def milestones_relation
+    @milestones_relation ||= if milestones.is_a?(ActiveRecord::Relation)
+                               milestones
+                             else
+                               Milestone.where(id: milestones.map(&:id))
+                             end
+  end
 end