How to slowdown the playback speed in FCP

Ian's suggestion of using Retime is probably best. Command-R the clip, select the downward pointing caret and select Slow > 50%, 25% or 10%, then you can do the same and select Normal to go back to full speed.

If you type the '/' key for a selection, the playback will loop until you stop it. [Selection can be a selected clip, selected group of clips, or a simple *range* selection...] Shift-/ will go back 2 seconds and play to 2 seconds past the playhead — when you stop playback, the playhead is returned to its starting position (where it was when you typed Shift-/).

HTH

Alternatively you could select all the clips involved and add the retime function, selecting any figure you wished.

Of course, you would have to remember to return the speed to normal before exporting your project.

The big question is why do you want to slow down the playback speed whilst editing?

I've starting developing a Slow Speed template/plugin and I would also be interested as to why you would need to do this as well.

My "plugin" (Effect) can reset the playback speed all the way down to a still frame (plus offset the start of the play range). This is a variation of one I've already published which allows you to keyframe an internal playhead (so playback frames in any order forwards or backwards... plus a few other features, but not really applicable to your request.)

Effects can be set up and then simply turned on/off as needed. No need to continually add/remove them while working.

Hold K and L together. Or hold the right arrow key.