I don't think that's a realistic view of the timeline. I've done features as multiple PRs and there are really two cases:

1. I can't submit pieces until I have the final version. PRs go up at the same time and can be reviewed one after another immediately.

2. There's a very specific split that makes that feature two features in reality. Like adding a plugin system and the first plugin. Then the first part gets submitted while I still work on the second part and there's no delay on my side, because I'm still developing anyway.

Basically, I've never seen the "but if each PR has a delay of 1 hour before a reviewer can get to it," getting serialised in practice. It's still either one time or happening in the background.