Instead of flagging > 24 hrs as a bad thing, flag < 1 hr as a good thing. If your board is supposed to be about engineering performance, then highlight actual performance and don't highlight negatives. Slow PRs are a problem to be discussed within the team, and often can be resolved with a simple request: "I just put up a PR that needs to be reviews so I am not blocked..." That would encourage people to help each other, instead of building a culture that say 23 hours to review a PR is fine because it isn't on the board.
We resolved this concern on my last team without breaking people's dev flow by formalizing a practice that every time you step out of flow or finish a task, you review PRs before getting back into flow. It worked great for that team. Focus on those types of solutions.
And as far as any other metrics, take the same approach, seeking the positivity. There are two sides to every story, so choose to measure the good side.