This sounds more like an issue with an app (or a kext) consuming way too much memory than any innate OS-level issue with paging. Activity Monitor and/or top can

This sounds more like an issue with an app (or a kext) consuming way too much memory than any innate OS-level issue with paging. Activity Monitor and/or top can be useful in figuring out what the heck is using up all that memory. Xcode can use a really surprising amount of memory sometimes but it's not going to just gobble up 20GB of pageable VM.