There could be two answers to this question -- fastest as in the fastest keyboard to type on and keyboard that's snappier to use overall. If you're implying to the former, every keyboard is good enough for faster typing. It all boils down to how efficient and experienced you are at typing and has little to do with the keyboard app itself. If you're implying to the latter question, Simple Keyboard is one of the lightweight keyboards on this list; hence, it should be snappier/faster than other keyboards.