this is the limits of a slow interface and 5 drives. See my other reply to enable faster pci speeds. because of how zfs works 5 drives is slower than 3, takes more cache and write speeds especially will be slower, quite a lot slower. with 5 drives and 16gb you can easily have a zfs cache of 12 gigs to help it along, i guess this is why you are getting large gaps between writes. as someone else said a pi doesn’t do well in this case but I reckon you can improve it. however as also said it is never going to be a speedy solution. secure and safe for data but not fast