How do editors guarantee the quality of peer-review?
I think you're making a mistaken assumption that your supervisor's not able to generate useful reviews quickly.  How long it takes to do a useful review depends on a few things: (i) the paper being reviewed, (ii) the kind of review being done, and (iii) the person doing the review.

Yes, if you're reviewing a long technical paper with lots of details it takes a long time to read through it.  It takes even longer if you want to check all the details for yourself.  It takes even longer if you're not familiar with these calculations.

I would assume that because your supervisor continues to get asked to do many reviews, the editors asking him find his reviews at least somewhat helpful. It's true that editors, at least in my field, have trouble finding enough good reviewers, but I can't imagine they would continue to ask someone who's review are consistently worthless.

I don't know what reviewing is like in your field (doing 20-30 reviews/month in my field would be unheard of) but here are some possibilities for how your supervisor could be doing many reviews quickly:


he's only selecting reviews he can do quickly and/or only reviews papers he wants to read anyway
since he does so many, he's probably gotten very efficient about doing reviews
he knows the field very well, so he can read the papers very quickly (and possibly assess it solely on the introduction, particular if he's going to recommend rejection)
he doesn't check low-level details, and primarily gives high-level reviews (which are still useful to editors, and depending on your area may be all that's expected)
he works really long hours