How do you speed up a clip in Final Cut Pro?

Right click > Speed

Right click the clip in the timeline and select Speed then set the percentage to a higher number.

If you want stop frame look you might try a new time line with a different frame rate then export out.

You might wanna activate “Frame Blending” when you’re slowing footage down.
It fills the gaps between frames up with fades of the surrounding frames.

Not the best of effect but it avoids that choppy feel.
(for more professional slower than slowmotion you can use plugins such as twixtor.)