Yes, because after 80% ZFS will consume CPU(s) in an attempt to find enough contiguous space for the new branch of blocks in order to prevent fragmentation. The fix is to either add more vdevs or to change the kernel tunable which controls the free contiguous space allocation algorithm. The latter approach is very likely to cause fragmentation where there would otherwise be none. As I don't remember the name of the tunable, you can search my comments and in one of them on the subject there will be a link to the procedure. However, I strongly recommend adding more logical units or physical disks as vdevs to the zpool instead.