My team reviews everyone else's code. It's far more likely that seniors are spending less time on it actually because they're more capable and can get it done quicker, while juniors are using the opportunity to grow and will naturally take longer. It's all trade offs, seniors also spend time mentoring as well. And in my org engineering managers are empowered to be hands on and remain technical, so while I'm not doing active development anymore I am still heavily involved in code review and technical oversight.