I agree with the other comments on here. Stansted passport control (and security) can be a nightmare, last time I passed through it took ~45mins and that was about the same time as you should be there.
We unfortunately have no choice which lane to use as we have a small child with us (so we can't use the eGates). But I did notice that the eGates queue was probably < 5mins most of the time. So if possible take your passport with you and use the gates.

Also be aware that stansted is also one of the worst UK airports for punctuality, ranking 19 out of 25 UK airports in 2017.