You have peer pressure to keep working longer, you get little done and you're wasting your brain power.
It makes sense if someone is junior and learning from the other.
Otherwise it's just an employers' tool to get people to squeeze in more hours of half-assed work (and then work more alone to actually get something done to have something to show for).
Async PRs reviews are way more efficient and less taxing on people.
When I did it (a long time ago) we worked for seven hours or so and went home tired but feeling like we actually accomplished a full day of work. Peer pressure was more about keeping to the normal schedule and taking breaks than putting in more hours.
But the startup died due to not paying enough attention to business issues. Programming productivity isn't everything.
Then I went to Google and my productivity got a lot worse, for many reasons but waiting on code reviews was one. To some extent I could compensate by making unrelated changes in parallel, or even working on a stack of patches and fixing up later patches when earlier ones needed to change. It felt like it was fighting a system that wanted to slow me down, though.
