Just wanted to say quickly, I’ve been playing (seriously’ish) with TrueNAS over the past 2 years and finally getting closer to the final config (though sad news to follow in a separate post, about electricity costs).
I just replicated from primary to secondary server and 1TB was transferred in 1 and a half hours. These are just initial test replications and following ones will be far smaller, but it’s still nice to see 
Config:
8 x 4TB RAIDZ2 (E5-1650v4)
TO
4 x 6TB RAIDZ2 (i3-9100F)
Maybe you’d want to share some more details, like the Network Speed and how you’re replicating?
If someone comes across this later on that might make it more helpful.
Very fair point, some other info for the future me!:
OS: TrueNAS Core 12.0-U8
Replication type: Using the built in replication, pulling FROM Server1 TO Server2, SSH transport
Server1: E5-1650v4 CPU on X99-E WS with 128GB Reg ECC RAM.
HDD’s: 4 x 6TB (1 is an Ironwolf, the others are re-used ‘wrong types’ (SMR), to be replaced as able
Boot: Patriot 128GB NVME
Server2: i3-9100F on Supermicro X11SCL-F with 16GB Unbuffered ECC RAM
HDD’s: 8 x 4TB Ironwolf
Boot: Mirrored SSD’s
Jail/Plugin: NVME (Samsung I think)
Network: MIKROTIK CRS305-1G-4S+IN (10G SFP+)
TrueNAS is just using the zfs send | zfs receive feature of ZFS, which is a serialized stream of the snapshot in question. Really fast and very easy even in the command line, also allows for incremental backups which is particularly useful with limited connectivity like WAN. TrueNAS GUI helps and offers automation as well.
One of my favorite features, because backup even with rsync or commercial backup solutions always was an underwhelming experience for me. Now I only need to remember to plug in device/drive for the scheduled task and everything just works.
This really is the reason why you want ZFS on all of your devices