30fps clip made slow motion in 30fps timeline
Twixtor is many peoples go to tool for retiming clips. The down side of retiming clips as slowmo, is that it adds extra time and steps to your workflow. Footage run through Twixtor should have much less artifacts than footage run through FCPX's optical flow.
I seem to remember reading that the trick to getting decent results out of Twixtor is to shoot at the shutter speed that correspond with your desired interpolated frame rate, e.g. 1/240th of a second for interpolated 120 fps.