Vegas Pro 14 didn't exploit the GPU in the same way that later versions do. Did you verify that your undisclosed hardware meets the minimum requirements for Vegas Pro 20? Perhaps you'd care to elucidate?
I don't know what your GPU or media is but it may be an issue with GPU decoding. 13 didn't have that and you can make 20 act like 13 by checking "enable legacy AVC" in preferences/ file io.