I was getting faster page load times than 1-3 sec on a 1 GB free tier AWS (Ubuntu 16.04) with 1.8 million posts. I’ve bumped it to a 2GB instance, and the page loads are a bit faster.
posts:rebake isn’t exactly snappy, but for a sandbox ok.
(I changed all 3 of the settings you mention in your first post - this made no difference whatsoever to page loads, or anything else as far as I could tell).
Is this an import? What does htop show?