Timeslips 2017 SR2 6 user very very slow on small business server 2011.

In particular the billing assistant - which this client of mine uses a lot. I installed to the C: drive on the server which is the system volume. This drive has way less space than the D: drive on the server - wondering if that is the issue? The space free on C: is several Gigs and certainly meets requirements, and they have 16G RAM.  Already excluded folders from virus scans. 
Any suggestions or ideas welcomed!
Thanks - Jane Giles