I found the same issue. For now, I either disable the gimbal, or set RC_SPEED=50 and restart Arducopter.
However, a lower RC_SPEED definitely makes the copter more sluggish.
See [Solved] Erratic motors - #26 by mikhail.avkhimenia which is very similar.
Let me know if either disabling the gimbal or setting a low RC_SPEED helps.