I find it is slightly slower (maybe 20%) than two individuals alone but the quality is quite a bit higher and the effect of this higher quality compounds over time (i.e. less tech debt -> fewer bugs, faster development on future code).

Im quite credulous of Kent Beck's claim that when categorizing the last ~15 bugs on a project with pairs and singles he found that all 15 were in code merged by an individual rather than a pair.

If it were an application you could just install I think everybody would use it. It demands psychological safety though, which most teams dont have, and is becoming less common these days.

It's funny because it starts to be (an application you could just install) - AI agents could work as pair programming buddies.