I think approval time would take much longer. The issue is that while actual time spent in review may be shorter, there's a lot of context-switching time costs that increase with the number of PRs submitted.
No one on the team is just sitting there refreshing the list of PRs, ready to pick one up immediately. There's a delay between when the PR is marked as ready and when someone can actually get to it. Everyone is trying to get work done and have some time daily in flow state.
Imagine you have a change; you could do it as one PR that takes 1 hour to review, or 3 small PRs that each take 15 mins to review. The time spent in review may even be shorter for the small PRs, but if each PR has a delay of 1 hour before a reviewer can get to it, then the 3 PRs will take almost 4 hours before they're done, as opposed to just 2 hours for one big PR.
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.