Ah kay, definitely not a RAM size problem then.

iostat -x 5 Will print out per drive stats every 5 seconds. The first output is an average since boot. Check all of the drives have similar values while performing a write. Might be one drive is having problems and slows everything down, hopefully unlikely if they are brand new drives.

zpool iostat -w Will print out a latency histogram. Check if any have a lot above 1s and if it’s in the disk or sync queues. Here’s mine with 4 HDDs in z1 working fairly happily for comparison: Here’s mine with 4 HDDs in z1 working fairly happily for comparison

The init_on_alloc=0 kernel flag I mentioned below might still be worth trying.