I got the impression that Swift Playgrounds was really Xcode in the background with most of the UI hidden. It certainly didn't feel snappy to me when I last tried it.
I’m not sure how to respond to “really Xcode in the background” but it’s an app with an entirely new codebase (that Xcode now shares some code with) and a full toolchain. If you haven’t tried the latest version released recently you should give it a try, it’s much improved from what we had in the past.
