Dragan Stepanovic did an interesting analysis [1] comparing lead and wait times times of async code reviews in contrast to pair programming.
His conclusion: "The optimal size of a Pull Request is one line of code which is reviewed immediately", i.e. pair/mob programming.
Disclaimer: I'm biased - founder of CoScreen [2] here, a pair/mob programming tool.
[1] https://www.slideshare.net/kobac/async-code-reviews-are-kill...