Thanks all for your comments. I was most nervous about coming to SQSP and losing search engine rankings. I've always known that the GTMetrix type scores are getting to be more important as the web develops, and I've really tried to up those, as traditionally these web builders don't do well there.
Frankly I'm REALLY impressed, the SEO in SQSP, it's smashed us right back up to page one, but the blogs that we are writing are really taking off; consistently getting fantastic traffic. Content is obviously key with google these days.
I never thought it possible to have a platform like this which is so easy to use and will actually rank so well.
One thing I've learned that you might like, is to use Cloudflare for your DNS, it really speeds up your web load time.