I find the streaming performance on the VFS alone to be quite good. Video opens in a couple of seconds, and never get any stutter at all even on high bitrate 4K. I would check if videos run ok outside of plex before assuming it's rclones fault. If they run fine outside plex but not inside then it's probably due to special considerations you need to have in the plex config. See Animositys recommendations on this (he has a post dedicated to his recommended setup including Plex).
What is really missing on the VFS currently is proper read-caching for the purpose of generally trading space for faster access to often used files. the VFS cache only has "full" mode which has horrendously bad drawbacks and I'd like to see a "readwrite" mode eventually that can cache partial read files while not forcing a full download and removing the ability to stream or have a pleasant user-experience. At that point I feel the old cache backend will probably be fairly redundant since it has a lot of other drawbacks that the VFS cache doesn't.
From a development perspective it also just seems wiser to add in the (relatively little) missing functionality to the VFS cache that is in active development by NCW than it is to do the much harder task of building upon unfamiliar code-base made by a developer who isn't active now and may not be coming back to continue the work.