As a follow-up to this, yesterday all my local sites started to run very fast again for no apparent reason! Even with the Query Monitor plugin running all pages load in under a second, some of them significantly so.
But, I’ve no idea why my local sites were running extremely slow under Windows 10 in the first place, or why they are suddenly fast again. Could they start running slow again without warning?
It would be very useful to have more tools to be able to diagnose problems. In my case the Query Monitor plugin was useful as it showed DB queries were running very fast so it didn’t appear to be a querying issue. But the page generation was huge. Sometimes a page was taking 15-20s to generate and render. I don’t know why this was.