That's the nice thing about 80MM and larger fans. They put out more air than 60MM or smaller fans while spinning at a slower rotational speed.
This equates to a quieter fan. So that we never have to endure anything like that dreaded 7000rpm black label delta fan that whinned like hell and came with metal finger guards so as not to ever cut any of your cables or you should you happen to be careless enough to stick your hand inside your machine while running.
Problem is everyone wants there PC to be quiet. So they try to make the fans as slow as possible while pushing as much air as possible. I am all for it. But I personalyl would rather see a 5500rpm 80mm fan on something like the Zalman.