You may have heard that Final Cut Pro X has a feature called "Optical Flow". When slowing down footage with Optical Flow, your footage will appear like it has been shot with a high speed camera.
Optical flow works by creating frames in between the actual frames of your footage. Instead of repeating frames or blending them together, FCPX creates new frames based on the content of your footage, thus making playback smooth.
Using Optical Flow
1) Select a clip in the timeline and click the "Retime" icon. Then select one of the slow presets.
An orange bar will appear above your clip indicating it has been slowed down (Orange = Slow, Green = Normal, Blue = Fast)
2) Click on the "Retime" icon again. Go down to "Video Quality" and select "Optical Flow".
Your machine now has some work to do, it's best to just let it sit for a while before you try to preview the results.
3) If you you'd like can slow your footage down even slower than 10%. Click on the retime handle (the vertical black line at the end of the orange retime indicator bar) and drag to make your clip longer. You'll probably want to zoom out before and while doing this to make room for your longer clip (Command+Minus). If you don't see the retiming indicator bar, select a clip and press Command+R to bring it up.