I would argue that the amount of time needed for a proper review exceeds the amount of time needed to just do it yourself.

When reviewing, you need to go through every step of implementing it yourself (understand the problem, solve the problem, etc.), but you additionally need to 1) understand someone else’s solution and 2) diff your solution against theirs to provide meaningful feedback.

Review could take roughly equivalent time, but only if I am allowed to reject/approve in a binary way (“my solution would not be the same, therefore denied”) which is not considered appropriate in most places.

This is why I am not a fan of being the reviewer.