Depends on site conditions and how you've got the machine set up.
In general, a fast sweep will give more depth on deeper targets, but will tend to lose shallower small or co-located targets. In trashy areas, go slow and listen carefully.
In general, a searchcoil motion rate of about 3 feet per second is a good match to the ergonomics of the T2 and most other FTP-Fisher metal detectors, and is also a good match to the circuit and software "motion" filters.