To improve the performance of the lists of users and groups You can try to disable the user/group deletion rule checking for the lists.
This will speed up the display of the list of users/groups.
Take note that the deletion rule will still be applied when you actually try
to delete the user/group.
Here is the corresponding improvement/option available since WorkflowGen
#3148: PERFORMANCE - Disable the deletion rules check for users, groups
and directories lists and form.
Just set the web.config parameter AdministrationEnableDeletionRulesCheck to