If I:

1. define a permission scheme granting the "JIRA Users" role the Browse permission:
2. in each project, map the 'jira-users' group to the project's JIRA Users role (this will be done automatically if 'jira-users' is configured as a role default member):

Then many parts of JIRA (notably dashboards) can run slowly if you have many users. With 45,000 users, a standard dashboard will take ~10s to render. If the 'jira-users' group is mapped directly to the Browse permission (logically equivalent), the same dashboard takes 300ms to render.