Just a cleanup of ~/Developer/Xcode/DerivedData every once in a while seems to keep it ticking over for me.
There was a time when the cache of simulators from every Xcode would build up and eat all you disk space just appearing under the amorphous grey 'System Data' categorisation under the 'About this Mac...' storage tab, but think they might have finally sorted that out.
I do still need to relaunch Xcode every few hours as it struggles with 100s of files open and suddenly I get a keyboard lag or copy paste refusing to work.
I'd definitely like to see something more lightweight for actual code editing, maybe de-coupling the SDK, and providing a decent LSP client that can be used with any language... but I don't see that happening anytime soon.