A friend is editing a two camera shoot using multiclip. The problem he's having is that there is no real-time playback when editing in multiclip no matter what he does. Out of complete desperation I drove to his place to investigate.
Looking over his project it all seems set up correctly but when you click on a clip in the Viewer it stops playback, playing via the Timeline or Canvas will not cause the clips in the Viewer to play but the thumbnails will update when playback is stopped.
Playing in the Viewer results in full-window playback of the currently selected (blue boarder) camera.
Digging around I noticed his clips are 720x402.
Suspecting that wasn't a multiclip compliant size somehow, I transcoded some clips (in the same codec) to 720x480 brought them into FCP, set the Canvas to "Correct for Aspect Ratio" and got better results.
Now, playback in the Viewer worked; it was no longer full-windowed but there was still no real-time. The Canvas would play or the Viewer, but not both concurrently.
Testing further I transcoded 60 secs of two of the camera angles to ProRes 720x480 and set Correct for Aspect Ratio. Now it all worked (well, the audio needed rendering and I didn't bother to check why); the Viewer and Canvas played in real-time together.
So the lesson here is frame size does matter. It seems that it must be a "standard" size and something listed (I guess) in the Easy Setup menu (Control + Q) to have real-time multiclip playback and editing.
I did lose Command + Z undo function on one test sequence but that seems like a FCP bug more than anything else. I did render the audio on this test sequence but Undo was still non-functional.
Looking at his Option + J (Button List) Keypad numbers are "Switch Video to Angle" while Command + (keypad number) is "Cut Video to Angle"