I agree with Lemo, your best solution is if you can not lower your spindle speed to a point were you can run your feedrate at a comfortable speed without burning or stalling is to look into a VFD or even better get a little spindle motor. As example I run a 3 flute cutters at a feed rate of 2to2500mm/min at 7000rpm or a 2flute cutter again from 2to2500mm/min at 9-10,000 rpm. Why 2 to 2500mm/min, thats were my router feels comfotable working, and then you have to adjust your cutting rpm to suite. Also i dont know what cutters your using but some of your standard router bits are just "CRAP" for CNC work, stick to good brands and spirel bits work best in my opinion.
Not adverising or sprooking for anyone homeshopcnc have nice little spindle motors, that will give yo an idea what to look for.
Cheers
katoh