I’m using a staging site set up to mimic the original site. The original site has some issues and is slower than I’d like so have been test GP premium. With GP premium the speed was about the same, some pages faster, but overall load reduced – a win even if speed didn’t win. Just updated to the GP beta.
Still overall improvement, but now 20% slower than the original wonky site. I can send screen shots from gtmetrix
I had to use their Australia server, which is slow. However, the numbers themselves look better?
That being said, if your site is already using Google Fonts and jQuery, there won’t be a massive speed difference at all. Existing sites also will have CSS caching turned off by default, which you can turn on in Customize > General.