I've made the same argument about the need for a programmer to not be disturbed. A colleague of mine has eloquently compared it to juggling: you start with one ball, add another and it gets complex over time. Disturb him and he starts over with one ball.

In my experience as a programmer I've grown to think that this argument only holds partial value. An observation that helped in this, is that programmers hate to be disturbed, but can always manage to be on time for lunch or beers on friday (it's a dutch corperate tradition to drink a few beers after the workday on friday).

I find myself, and others, to be capable of reviewing a PR within an hour. I just need to be aware that when I'm done with a coherent unit of code, that I should switch to review mode and check if there are outstanding PR's. To reuse the metaphor: I've stopped juggling and can choose which set of balls to pick up next.

So I do think it is a cultural thing. You need to commit yourself to have reviewing be a part of the work process. I find IM services and PR's to cater perfectly to the way the usual programmer works. He can ignore the message while he's mentally keeping 5 balls in the air and look through his messages when he's ready to start his next activity of the day.