Okay, if it's that slow then parasitic inductance on your gate drive leads (and the rest of the converter in general) is probably the main cause. Make them as short as possible (like a few inches at most), and use twisted pairs to reduce inductance. Shrinking the area of the loop formed by the input bypass capacitor, MOSFET, and catch diode will also reduce stay inductance and reduce ringing. That should help significantly. But if you want switching times of 100ns or less you'll probably need a tighter layout than that, with everything on the same board.