Code Review Velocity – Need for Speed
medium.com
medium.com
I'd personally argue that the most important metric is "time to consensus". There are sometimes good reasons to hold off on merging an approved PR (external effects) but that should not reflect poorly on how your team does code review. I define consensus as when the author and the reviewer have resolved all blocking conversations.
That's the model I've used on CodeApprove (https://codeapprove.com) which focuses somewhat obsessively on unresolved conversations as the key element of a code review.