It may have improved, but a few years ago at least zxcvbn was implmented by repeatedly trying various password "simplifications" at some kind of entropy cost.

It may have improved, but a few years ago at least zxcvbn was implmented by repeatedly trying various password "simplifications" at some kind of entropy cost. At the time it was quite easy to construct long (something like 60-char - our limit at the time) pathological passwords where a single zxcvbn check would take at least a minute of CPU time, with strongly super-linear growth for each additional char. Might have been improved by now; not sure. If it's not you might be wasting electricity another way ;-).