From Longman Dictionary of Contemporary Englishnot so fastnot so fastspokenNOW used to tell someone not to be too eager to do or believe something Not so fast. We’ve got to prove it first, haven’t we? → fast
Examples from the Corpusnot so fast• Both were expanding their departments as well, though not so fast as Ranieri.• Not so fast, guys. One win doesn't make a championship season.• But not so fast, or so the town of Oro Valley decided August 30.• It's also big, but not so fast or user-friendly despite its curious system of organizing bits into folders.• Not so fast! You'll scrape the paint!