Typically, you wouldn't bother manually reviewing something until the automated checks have passed.
I'd rather hop in and get them on the right path rather than letting them struggle alone, particularly if they're struggling.
If it's another senior developer though I'd happily leave them to it to get the unit tests all passing before I take a proper look at their work.
But as a general principle, please at least get a PR through formatting checks before assigning it to a person.
Let them finish a pull request before spending time reviewing it. That said, a merge request needs to have an issue written before it's picked up, so that the author does not spend time on a solution before the problem is understood. That's idealism though.
The earliest feedback you can get comes from the compiler. If it won't build successfully don't submit the PR.