

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:
The init_on_alloc=0
kernel flag I mentioned below might still be worth trying.
It’s called secure boot and it’s been around for over 10 years now.