Usually at most 2-3 engineer have enough context to fully understand what your code is doing.
Needing full human attention on a co.plex task from a pro who can only look at your thing has a wait time. It is worse when there are only 2 or 3 such people in the world you can ask!
Not saying this is a good situation, but it's quite easy to run into it.
Most excellent.
Most devs set aside some time at most twice a day for PRs. That's 5 hours at least.
Some PRs come in at the end of the day and will only get looked at the next day. That's more than 5 hours.
IME it's rare to see a PR get reviewed in under 5 hours.
If you work in a team of 5 people, and each one only reviews things twice a day, that's still less than 5 hours any way you slice it.