Whether you play a clip faster or slow-mo it, Final Cut Pro does not change your source media file. To achieve a speed effect, Final Cut Pro either skips frames or repeats and blends frames together.
Fit to Fill lets you cram or stretch a clip to fit into your sequence without having to do the math.
1 Either mark In and Out points in your sequence or cue the Timeline playhead over a gap/clip that you want to fill/replace with a Viewer clip.
2 Load the source clip into the Viewer, and if necessary, mark the clip’s content with In and Out points.
The entire content of the source media will be used if no In or Out points are marked—no handles are necessary.
3 Patch the Source and Destination tracks to target the sequence ...