Merge requests too slow/delayed

We have a problem where the merge request becomes very slow, and in some cases reaches 20(!) minutes. The problem is phantom, not regular.
We tried to find the problem in the logs, but found nothing suspicious. When this happens, monitoring also does not show resource overcommitment by cpu or memory.
We have about 15-20 active users. Repo in which this happens most often has a code volume of about 100mb.

Our GitLab CE instance is installed from official rpm repo on CentOS 7.
GitLab version 15.8.1 (regularly updated)
CPU: 8 cores
Memory Usage: 11.6 GB / 15.5 GB

We would appreciate any help in solving the problem.