How fast does the stealth fighter plane go?

Transonic speeds only, below mach 1. This is because its shape is very unstable, and going past mach 1 would mean essentially encountering too much compression that the shape of the plane by nature can't deal with and would be uncontrollable. (The same reason why it is invisible to radar.) That said, a sonic boom isn't quite ninja stealthy. It could go mach speed in a steep dive perhaps, but the pilot would evidentally have no control over the aircraft.

So less than Mach .98