After some googling:

Some Linux distributions (at least Debian, Ubuntu) enable init_on_alloc option as security precaution by default. This option can help to prevent possible information leaks and make control-flow bugs that depend on uninitialized values more deterministic.

Unfortunately, it can lower ARC throughput considerably (see bug).

If you’re ready to cope with these security risks 6, you may disable it by setting init_on_alloc=0 in the GRUB kernel boot parameters.

I think it’s set to 1 on Raspberry Pi OS, you set it in /boot/cmdline.txtI think.

[Exhaustive ZFS performance tuning guide](openzfs.github.io/openzfs-docs/Performance and Tuning/Workload Tuning.html)