Complex sentence: If the zpool consists of only one group of disks configured as, say, eight disks in raidz2--then the write IOPS performance will be that of a single disk, however, read IOPS will be the sum of eight individual disks.
Split into simpler sentences: If the zpool consists of only one group of disks configured as, say, eight disks in raidz2, then the write IOPS performance will be that of a single disk. However, read IOPS will be the sum of eight individual disks.