Investigate which parts of the MergeRequests::AfterCreateService are slow

Background

We want to improve the performance of NewMergeRequestWorker which calls the MergeRequests::AfterCreateService. In #417972 (closed), we'll be adding metrics to determine which parts are slow.

Proposal

Once #417972 (closed) is done, read the data and devise on a plan on what we can do to improve them. We can start to think to split the service into smaller workers, or if there are specific parts that we can fix without doing much refactoring, etc.

Result is to create issues and add it to the epic of this issue for the plan we want to take.