Every OS is "blazing fast" upon clean install. The slowness, beachballing and issues usually appear after longer use.
Yep.
Usually caused by lots of cache files (user caches, system caches, etc.), here and there a faulty .plist, corrupt "open-wth" list... etc, etc...
... which all result to lots of disk activity, which causes slow-responsiveness of the Mac.
It does help to use a fast SSD.