Basically if you are dealing with compressed formats, like R3D, they need CPU/GPU/Red rocket power to decode that efficiently. Uncompressed formats, like DPX sequences, take a lot of space and need a fast drive instead of a beefy CPU because there’s not much to decode.
I have a Pegasus Thunderbolt RAID at work and it reads/writes about 500-700 MB/s. I believe you can get close to 500 MB/s on a fast USB 3 RAID, that should do fine unless you are dealing with let’s say uncompressed 4K or you’re trying to make a multicam edit with a LOT of angles.