Uh... wow. https://bugzilla.mozilla.org/show_bug.cgi?id=1845775#c14 "Long story short: the watchdog thread interrupted us four times, so we gave up and threw an error. This also explains why Chrome is significantly faster here: when interrupted, we stop the regexp, handle the interrupt, then restart regexp execution from the beginning, whereas V8 does scary code crimes to keep the regexp running despite the possibility of triggering a GC with unrooted values on the stack. (The words "manually relocate unhandlified references" are involved.)"

That doesn't sound good...