Cluster recently upgraded to OCP 4.12.19 experiencing serious slowness issues with Project>Project access page.
The loading time of that page grows significantly faster than the number of entries, and is very noticeable even at a relatively low number of entries.

4.12.19

Easily 

1. Create a namespace, and add RoleBindings for multiple users, for instance with :
$ oc -n test-namespace create rolebinding test-load --clusterrole=view --user=user01 --user=user02 --user=...
2. In Developer view of that namespace, navigate to "Project"->"Project access". The page will take a long time to load compared to the time an "oc get rolebinding" would take.

0 RB => instantaneous loading
40 RB => about 10 seconds until page loaded
100 RB => one try took 50 seconds, another 110 seconds
200 RB => nothing for 8 minutes, after which my web browser (Firefox) proposed to stop the page since it slowed the browser down, and after 10 minutes I stopped the attempt without ever seeing the page load. 

Page should load almost instantly with only a few hundred role bindings