Is the workstation / machine for User2 same (same configuration) as that of User1 ?
I don't think the slowness is associated with security unless the report does not open at all for the non admin user?
Are they logging in from the same location or are the users at different physical locations?
For test,can you have the admin user login to the same PC as the no admin user and validate the time taken for report run?
Is this an issue with one report only or is this with all or multiple reports?
Let me know