Well, that would certainly be alarming if true, but I don't think it is. As far as I can see, the overhead of making the visibility map crash-safe is just (1) a very small percentage increase in the work being done by VACUUM and (2) a slight possibility of extra work done by a foreground process if the visibility map bit changes at almost exactly the same time the process was about to insert, update, or delete a tuple.
Let's forget the overhead posed by vacuum. Can you please point me to the design which talks in detail of the second overhead?
