Imagine you have a PR with several files changed in it.
During review, you make the comments about the changes that will cause you to decline the PR.
The developer fixes the issues and then resubmits a new PR.
You now have to go and examine all the files that are unchanged between the two PRs all over again. It may not be the same reviewer.
It would be REALLY useful to be able to mark files/diff chunks as reviewed, and, if the chunks remain the same between reviews, they are collapsed in some way.
The constant scrolling up and down looking at things which we have no way of ignoring once we've reviewed them, makes it just a little painful to do.