4K Workflow in FCPX - crop down for multicam angles
I'm a little confused.  Perhaps it's the terminology you're using.  Are you really talking about cropping (throwing away the top, bottom, and sides of the image), or do you mean scaling?  Are you creating three angles from the same camera (which explains why you'd crop, but I can't imagine happening in an interview scenario), or do you have one BMPC and two other 720p cameras (which would explain why you'd scale the BMPC)?  

In either case, you should probably skip the compound clips.  FCPX doesn't play nicely with CC's stacked inside of multicam clips.  Just make sure that when you create the multicam, it's the size of your desired output.  When you add higher-resolution footage to a lower-resolution multicam clip, FCPX scales the new clip down to fit within the smaller bounds automatically. But you can always use your transform tools to scale the image up afterwards, effectively cropping it.  There's no speed or image quality penalty for this type of operation because FCPX doesn't apply these transformations sequentially, it does them simultaneously at render.  For this reason, it's usually good to keep your multicam output size the same as your smallest angle.

I also wouldn't render any corrections beforehand.  Each render you do is going to degrade the image quality through compression and/or error.  If it's the NR and color correction that's slowing down playback, you should just disable them while you edit and re-enable them before you render.  If your computer bogs down with vanilla 4k prores LT, you could try working with proxies, but you'd save yourself some headache by just upgrading your computer.