The only somewhat similar issue we’ve seen reported involves long hangs after doing certain things.
Nine times out of ten though these kinds of issues are too contextual to explain them in just one response. There is no one answer, and it could be anything from an out of date utility you use that doesn’t work well on the new system and is slowing programs like Scrivener down, to a specific view setting you are using (that Composition works fine might point toward that), etc.
You just have to play around a bit rather than looking at one single thing. Use the Interactive Tutorial, see if it is slow. Try running Scrivener with nothing else running but Finder, etc. Turn off the more complicated editor settings that might not be enabled in Composition, like Page View and line numbering.