I've spent the past decade working at a company that requires all changes to be reviewed no matter how trivial. Changes can be reviewed after the fact if the author marks it "to be reviewed". Doing that is rare, like <1% of changes.

I have never found this policy to significantly hamper my overall productivity both as an author or as a reviewer.

So have I, except it's been three companies. One of them, ThoughtWorks, had built-in code review in the form of pair programming. I don't really have data on productivity, one way or the other, nor a strong opinion on the desirability or benefit of mandatory review. I was just addressing the specific comment of "Not much time is lost" above. Ç'est la guerre.