I find that RAIDZ rebuilds almost as fast. Like my 75% full RAIDZ2 pool of 10x14TB disks resilvers a disk in about 18 hours. Which means that it resilvers the 10.5TB of used data onto the new disk at an average of 160MB/s, which is not much slower for the average write speed of the disk by itself. Maybe a mirror would rebuild in like 15 hours? Anyways it doesn't seem like a meaningful amount faster and the much higher capacity of RAIDZ2 over a bunch of mirrors saves a ton of cost both in numbers of disks, but also in slot cost and interface connection costs.