In my personal experience, the hump from wide-eyed novice junior to productive contributing member really doesn't take that long when every line that everyone writes is code-reviewed. The first couple of code reviews might take longer than usual. But it definitely much less time than it used to take to bring a junior up to speed. No net productivity in the first year used to be a rule, when bringing juniors onboard in the dark ages of computing. With 100% code reviews, that process seems to take no more than a couple of months -- maybe even as little as one month. The difference is dramatic.

It's a fabulous way to transmit not only good culture, but also love of our craft.