GroupsController#merge_requests P90 is above 500ms

According to this Kibana visualization, the GroupsController#merge_requests action has a P90 of 1.239s (above our 500ms target) for that last 7 days as of this writing (2021-03-23).

It's also noticeable that P90 db_count is around 153 and P90 db_duration is 0.621s. If we can reduce the time spent on DB, we can reduce the response time of this endpoint as well.