pulsed current with 200 kHz squarewave signal
You have a massive power MOSFET with a lot of capacitance. If you reduce the size of the MOSFET you can go faster, or change the type to a different technology than silicon. 

A cheap one that should show a lot (like 5 or 10:1) improvement is the FDD1600N10ALZ.

They're cheap enough, but you can run some simulations to see what the predicted performance is. You can also try reducing the 600\$\Omega\$ if that's not enough improvement.