Projects::MergeRequests::CreationsController#create P90 is above 500ms

According to this Kibana visualization, the Projects::MergeRequests::CreationsController#create action has a P90 of 1.173s (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 175 and P90 db_duration is 0.334s. If we can reduce the time spent on DB, we can reduce the response time of this endpoint as well.