Backup ZFS extremely slow.

I weekly backup my stuff to a Pentium 4 HT on FreeBSD 13.0 and to This i5-2520M. FreeBSD on the Pentium is faster than Ubuntu 21.10 on the i5; The Pentium runs constantly at a ~92% CPU load on one thread and it succeeds in transferring the stuff at ~22Mbps. The I5 is slower and sometimes transfers at 80Mbps, but it also stops transferring for many seconds. When it stops transferring one of the CPU threads has a 100% load according to the load measurement of Conky. If I look with the task-manager the load is normal and in the expected range and it never exceeds for any thread more than 70%.

All zfs datasets are lz4 compressed on all 3 sides and I use the same send and receive command only the dataset names are different :) The send command is used with the -c parameter. The same situation worked fine with ZFS 0.8.

I have the unhappy feeling that the 100% load on one thread is maybe caused by emulation of an instruction not supported by the i5 and that is why conky measures it and the task-manager not.

I hope you can find it or maybe 2.1.2 is better, otherwise I might have to move the i5 to FreeBSD too :) :(

Here I use a modern 2TB HDD and on the Pentium 4 HDDs in two pools; one with 2 x 3.5"IDE and th other with 2 x 2.5" SATA-1.

ProblemType: Bug
DistroRelease: Ubuntu 21.10
Package: zfsutils-linux 2.0.6-1ubuntu2.1
ProcVersionSignature: Ubuntu 5.13.0-28.31-generic 5.13.19
Uname: Linux 5.13.0-28-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu71
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Sat Feb 19 19:16:53 2022
InstallationDate: Installed on 2021-10-30 (112 days ago)
InstallationMedia: Ubuntu 21.10 "Impish Indri" - Release amd64 (20211012)
SourcePackage: zfs-linux
UpgradeStatus: No upgrade log present (probably fresh install)
modified.conffile..etc.sudoers.d.zfs: [inaccessible: [Errno 13] Permission denied: '/etc/sudoers.d/zfs']