also, sending the clip with the slowdown applied to compressor and changing the blend mode to optical flow usually noticeably improves the quality of the speed change. Not sure if this would have any impact on the quality of the key. Probably a good idea to do a test.
I've had good luck down to 20-30% speed using Shake speed change in adaptive mode. The same basic capability is available in Motion and Compressor but it's called Optical Flow. It actually predicts and generates intermediate frames rather just duplicating.