Gitlab UI slow after upgrade to Version 13.8

we use a self hosted Gitlab instance. We used Version 12.2 for a long time and are now in the process of upgrading. We keep to the upgrade steps recommended by Gitlab. After the upgrade to version 13.8.8 the UI became significantly slower. Especially loading details of a project or a merge request can now sometimes take 3-4 seconds while it was without a significant delay before. Loading the url by wget is still very fast. After the upgrade much more bundles are loaded and the rendering only finishes after the last ones have arrived. For example, when clicking on the project name in the projects list only after a graphql request has been made and a bundle called “MrPopoverBundle” has been loaded the UI becomes interactive. Between those requests is often a relative long time of inactivity where the Frontend seems to be busy. I have used Chrome Lighthouse and the Firefox Performance Profiler, but no conclusive hint. There are no errors in the Puma log and the sidekiq Dashboard also shows no hanging processes or something like that.

Another behavior is when creating a new merge request, the widget that checks if it can be merged displays a spinner basically forever which only disappears after reloading the page.

Example output of the performance bar when loading projects page (size of repository: 2.8G):
19ms/45 pg 0 gitaly 5ms/32 redis 0 es 0 external 29.3MB memory 433|528|1546 / 40 total

Loading merge request detail page:
18ms/55 pg 0 gitaly 4ms/29 redis 0 es 0 external 33.6MB memory 411|490|2041 / 70 total

Hardware: 4 CPU, 8G RAM, 4 Puma workers (increase from 2 had no effect).

Hope someone can help, thanks!

Since 13.8.8 is no longer supported, and the latest release being 16.10.3 I would suggest continue upgrading your server following the upgrade path until you reach the latest version. There is every chance if a performance regression was introduced, that it got fixed in a later version. It makes zero sense to debug such an old version, when there are newer releases with a lot of fixes, including critical security fixes.

Also, some 13.x versions were vulnerable to people exploiting with dropping crypto mining executables on the server and had high CPU usage because of the vulnerabilites being exploited. Another reason to ensure you are on latest version.