Inspired by this question (including a comment by @JeffE that a paper should be short to be reviewed within a few hours) and some other questions on this website.
Almost everyone believes that peer-review of a paper takes about 1 day and peer-review of 20-30 papers per month means low-quality reviews.
My supervisor receives 50-100 peer-review requests every month (I know because I organize his mailbox) and accepts 20-30 papers submitted to high-impact journals (say impact factor 10) to review. He never asks us to review (contrary to many comments) and he does the review personally. I do not have exact statistics, but based on his schedule, he shouldn't spend more than 1 hour to review a paper.
My question is: if his peer-review is careless and of low-quality (as assumed as a fact by many here), then, why do editors frequently invite him to review? Note that he is one of many.
If the editors do not consider the quality of reviews and simply invite famous people frequently. Then, there should be a flaw in the quality of high-impact journals.