Some time ago the kernel devs decided to GPL the FPU instructions in the kernel causing incompatibilities with ZFS and some major performance degradation especially on raidz, checksumming, and encryption. Anything basically using   AVX/AES-NI. Here is a small article on the situation:

https://www.phoronix.com/scan.php?page=news_item&px=NixOS-Linux-5.0-ZFS-FPU-Drop

I had heard that this was backported to all the LTS mainline kernels, but have not been able to find out for sure if this is the case.  So far only 5.0+ kernels.

I would like to add the small patch to debian-sources-5.2.6_p1. It applies cleanly and I was thinking to only apply it when using the zfs USE flag, but I wanted your opinion/go ahead first. Thanks