If you are slowing clips in FCP you are by default using frame blending. Below 50% or so it starts to look nasty.
Motion, however has more to offer....optical flow!
When you send a clip from your timeline in FCP to Motion original timings are transferred, including variable speed. Make sure the embed option is checked.
In the timing drop down in Motion's inspector there's a drop down for frame blending.
Blending...which is what FCP uses
Motion Blur… Continue