Unless you have a broader context, reviewing 300 line PRs in 5 minutes is going to be surface level at best. Plus that time comes with an expensive context switch so the actual cost is likely more like 20 - 30 minutes. At this point, I think a reasonable question is why not just use AI for shallow reviews like this? This would free up bandwidth in situations where you really want code reviewed by someone else.
One of the advantages of small, frequent code reviews is that a team shares the broader context.
Which is far more valuable than ten minutes of extra typing.
There are better ways to get broader context. Such as getting people to work in other areas of the code. A 5 minute review isn't going to do much in this regard.