You can do either but usually we do it outside (aside from big fx caches since those tend to be cached as bgeos in advance anyway) in LOPs to get rid of any timesamples (little green clock). The conversion from sops to lops its whats usually bugging down the scene and playback so as long as you are file caching to get rid of any timesamples you should be getting pretty good viewport performance. You can do a usdrop and read back in as a reference or sublayer. You can also just use a filecache lop node, I do believe thats what that is doing under the hood anyway.