Submitting merge request review is slow

Summary

Submitting merge request reviews with multiple comments is slow, sometimes taking minutes.

Steps to reproduce

1. Open a merge request, and add multiple comments
2. Submit review

Example Project

Particularly reported on www-gitlab-com release posts.

What is the current bug behavior?

Slow to submit (multiple minutes)

What is the expected correct behavior?

Should submit in less than 10 seconds

Output of checks

This bug happens on GitLab.com

Possible fixes