I'm not that familiar with ZFS, much more familiar with mdadm, but with 4 500GB drives and raidz3 I expected a 500GB volume with three mirrors, but instead it looks like 1TB volume.
I wonder if the install process for some reason decides to write all blocks since it is raidz3? That seems a bit odd thought, but it could explain the time?
Let's not forget. raidz is just a type of organising the vdevs one way or another. If you put aside the technicalities of how many disks for parity, how many you can lose before losing the pools, which one raidzX is faster to read or write, all that doesn't matter really for such a small number of physical devices and such a generic pattern of reads and writes, as oppossed to say databases. Read: any choice of raidz for this is good for "speed".Then you have the zfs filesystem on top. Again, for OPN, is in terms of speed, almost as fast/slow in magnitude as the underlying physical media. Yes mirrored is faster than not. NVMe faster than SSD, etc. All in all, same ballpark.