I did not say it was specifically the entrance exams. In most cases there was a strong socio-economic component.
In my very strong School we still have excellent students reluctant to apply to Oxbridge, particularly Oxford, for doctoral studies because school friends who did undergraduate degrees there were miserable. My sense is that there is usually a socio-economic aspect to this. Not, of course, that one need be middle class to fit in; but it seems that a significant percent who are not do feel out of place. It is difficult to tease out to what extent this may be a self fulfilling prophecy.
In no case can I imagine that theRussell Group maths professors, leading researchers, etc, I am thinking of would have been stressed by the Oxford workload, or unable to keep with peers.