KnownHost. Blimey, that is inexpensive!
I am not sure what my bottlenecks are. But one I have considered is DNS delay, so I was thinking maybe DNSMadeEasy might help. What do you think? As I understand it, it acts a bit like a CDN where the POPS are spread around.
You have obviously done a lot of all this speed tweaking yourself! It is one of the more enjoyable aspects of running my forum.
I think the loading of CSS is done upfront, as my CDN does not cache that, I believe. I wondered if Redis helps with that.
I use CloudFlare for DNS, never tried a premium DNS service. I think the savings would be mere milliseconds.
CloudFlare can cache CSS is you add a page rule for css.php* - as for any other CDN, I'm not sure. Redis won't cache it.