This caught me by surprise, so it’s worth a shoutout. This setting is default ON, but I suspect most sites don’t use it. I watch the log on a bigger site stuck on “HTML post processing” for several minutes and thought … what? I have not enabled any post processing. Turned this off, and had a massive speedup:
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This feature is about two days old, that’s why you were caught off-guard 
https://www.netlify.com/blog/2020/10/30/netlify-changelog-october-2020/ (a note just in a list of other changes)
OK, so the “toggle” is new, but feature is not.