What settings do you have in PVS (Picasso Varibale Setter)?
You should use Virge.card/chip etc always from the Mediator Archive and overwrite the original Picasso96 ones... use Picasso 2.x always, don't use the 1.x versions.
The only monitor you have in devs/monitors is the Virge one, right?
Try to turn off the caches to see if graphics run at the same speed. If you notice that the graphics are running dead slow maybe it's because the cpu is working doing stuff that should be done by the gfx card or because you have enabled any strange option.
You should have disabled: the "debug" option,
And enabled: disable amiga blitter, planes2fast, keep amiga video 31khz, no planar to chunky blits, blitter...
well, tell us your PVS settings
BTW, I installed a Virge with P96 in a friend's A4000T Mediator without problems.