Upgrade from 7.14 to 8.x slows down Sidekiq jobs: issue with commits count in repositories?

I noticed that since I upgraded to 8.x two weeks ago (from 7.14) merging may take also ages in my case (a few hours), and that the web app is not properly updated.

The reason for that is that some Sidekiq jobs take hours to complete:

2015-11-24T19:33:53.172Z 1675 TID-ouioxol88 PostReceive JID- INFO: done: 3231.524 sec
2015-11-24T20:20:19.285Z 1675 TID-ouiovs6f4 ProjectCacheWorker JID-6462f97941c43b42b706edc9 INFO: done: 5572.114 sec
2015-11-24T20:35:47.984Z 1675 TID-ouiov013o PostReceive JID- INFO: done: 6481.818 sec
2015-11-24T20:51:16.743Z 1675 TID-ouioxl488 ProjectCacheWorker JID-7b3d7d458f711bc3e1a0c325 INFO: done: 6500.514 sec
2015-11-24T21:06:45.442Z 1675 TID-ouioxa0l0 PostReceive JID- INFO: done: 7126.051 sec
2015-11-24T21:22:14.153Z 1675 TID-ouiou12mo PostReceive JID- INFO: done: 7473.421 sec
2015-11-24T21:37:42.849Z 1675 TID-ouiox3e4k PostReceive JID- INFO: done: 7993.463 sec
2015-11-24T21:53:11.548Z 1675 TID-ouioxol88 ProjectCacheWorker JID-12780dfd5e608dfc4622b146 INFO: done: 8358.296 sec

As suggested by @stanhu in the ML, it might be related to counting the number of commits in the repository.

I maintain a gitlab instance for our university. We have 685 projects and 537 users currently. Most projects are students projects, so they do not contain that many commits.