You need to move the machine fast enough to maintain an adequate chipload, and spin the endmill slowly enough that even when it slows down into a corner it won’t overheat and begin to melt the plastic.
See https://precisebits.com/tutorials/calibrating_feeds_n_speeds.htm for a testing technique
You’ll also want to clear out as much as you can using conventional endmills — just use the ball where needed for undercuts.