4K Workflow in FCPX - crop down for multicam angles Shot an 80min interview @ 4K 422LT (271GB)on BMPC and want to create 3x 720p angles for multicam. Unsure if it's best to create them as compound clips cropped from the one 4K master clip inside of FCPX or Render out the 720p's as standalone clips then reimport.

Once the Blackmagic ProRes422LT 4K clip has been imported and a LUT + Noise Reduction applied it gets unwieldly (to say the least)

Is it best practice to Render out a new 4K clip via Compressor with those corrections baked in, OR

Create cropped 720p compound clips (from the effects applied 4K) for each of the angles for use in Multicam, OR

Crop the 4K down to 720p angles and Render those out via Compressor as individual clips and reimport to make multicam angles on interviewee.

If there is 1x 4K master file that has corrections (LUT+NR) applied - would it be better to render out a new 4K version with those changes baked in, and then make 720p multicam angles from the corrected clip - I guess that's my question.

Always looking for ways to make everything work faster.

2013 MacPro 28GB RAM / 30TB LaCie T2 RAID - not sure what upgrade options are available.
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.