In physics at least, it's been my experience that even though the percentage of women physics professors is low, almost every one is an above-average researcher. They are certainly harder working on average. Meanwhile, there will be several men in the department who have just coasted. This isn't among new profs, but among those who have been around long enough that they definitely experienced gender discrimination in hiring. I don't think this is a biological thing, just an interesting side effect of strong gender discrimination, where the women who make it in have to be exceptional to do so.