ZFS network send/recv very poor performance

I have a partially proxmox specific question.

We have two backup servers running PBS current version.
We make SMB (samba) backups are made to first server.
Second server is at a different location (geo redundant)
We use ZFS file system.
On the primary server every day is taken ZFS snapshot.
We send incremental snapshot to secondary server every day (ZFS send, recv) over SSH.
it's a completely basic ZFS backup system.

But ZFS sending/recv is very very slow!

server specifications:
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