Regarding hosting, yes I have. I started by reviewing which plugins where executing lots of SQL queries , using lots of memory, and adding to the load time on either the frontend or backend. This great WordPress plugin was a lifesaver for that: https://wordpress.org/plugins/usagedd/
I managed to cut load time by about half with that. I was using a VPS, not the cheapest shared hosting so I didn’t think I had that much to gain there. But I gave Kinsta a go and BAM – the site got 4 times faster again. And that’s the speed difference you are noticing. The site was super fast on Kinsta even when I copied the version of the site that preceded my painstaking code optimisations. Perhaps I didn’t need to bother. Although having said that, Kinsta have certain plugins that are on a blacklist for being too resource intensive. You can view that as a drag because you may have to swap one plugin you’ve been using for a long time for another similar one, but I see it as useful advice on how to maintain a speedy site from people who know what they’re talking about.