Slow TTFB for regular domain users, fast for admin accounts

Hi.

I run an IIS server which started exhibiting slow performance recently. We outsource our infrastructure, and after spending 2 hours ruling out all kinds of issues, this is what we discovered:

1. All "regular" users on the domain see a TTFB overhead of about 4 seconds for all HTTP and XHR requests
2. If you log in as an admin account (as set up by our third party manager), the overhead disappears
3. This was behavior replicated on a user desktop and on a domain controller which resides on premises

To be explicit, TTFB for regular users is 4 seconds slower than for admin users.

We seem to have ruled out security protocol and cypher issues. Could it be related to proxy settings or or Windows Firewall settings? The network admin says that the difference between regular and admin users relate to things like disabling USB drives and other corporate policies, nothing network related.

Any help would be appreciated.

Thanks!