DIAGNOSIS The turntable motor is likely failing. It's a common wear part. A slow spin is a classic symptom of a dying motor.
FIX Order part WR55X10922, the turntable motor assembly. Unplug the unit. Remove the glass tray and roller guide. You'll see the motor mounted in the cavity floor. Disconnect the wire harness, remove the mounting screws, and replace it.
IF THAT DIDN'T WORK Check the roller guide and glass tray for cracks or warping. A warped tray creates drag. Replace the tray and guide if damaged.
HEADS UP The motor is not serviceable. Do not attempt to oil it. The new motor will click when it rotates. That's normal.