Access to pages is slow under high load on the security cache

Steps to reproduce:

Unclear, the key conditions seem to be a high write load on the security cache, caused, e.g., by a lot of notifications being dispatched with a high user count which causes rights checks for all of them whose results are inserted into the security cache. At the same time, a high number pages are viewed and a group containing most users is updated, causing the security cache to be emptied. To really trigger issues, the security cache should be large (like 1M entries) and full, so the invalidation actually takes a few seconds.

Expected result:

Accessing pages, in particular pages that are frequently visited, is still fast.

Actual result:

Loading any page takes 20-30 seconds.