can't maintain ssh connection to perform large zfs send

I'm trying to zfs send a 6 TB pool from some old disks to some new disks. It gets a few 100 GB through, but then the ssh connection always drops with:

client_loop: send disconnect: Broken pipe

I've set ServerAliveInterval in my client's ~/.ssh/config but that doesn't seem to help. Could it be the TrueNAS dropping the connection?

I've noticed also that the zfs send/receive consumes 100% CPU, I guess because of decompression/recompression?

Thanks for any suggestions...