Turns out having years of history, extensions, and a multi-gigabyte cache makes any browser slow.
It's a real pita for the same reason. Needed to look at browser history older than a few months and discovering it's all been deleted (by Firefox) was a very unpleasant experience.
Seems to be a needless data loss "optimisation" (sic), but I have no idea what the real world use case for that would be.
I suspect they need to do this to keep history searches fast, since I also separately hacked a bunch of `about:config` options to retain more history and they run quite slowly now, particularly on my phone.