Wow, switching to
http made quite a difference! By rough calculation, time to do a continuous replication is about 1/4 of what it previously was.
My XOA is connected to a 1Gbps interface and so are the various XCP servers. This is a dedicated network for VM server management / VM replication.
As comparison, a 1.76TiB VM took 11 hours 28 min to do it's initial replication when running on
https. But once
https was turned off, a different VM which is 4.28TiB, took 19 hours and 58 minutes. It was the first time I was able to do an initial replication on this VM as previously, it would take more than 24 hours and XCP would kill the job.
Smaller continuous replication jobs (deltas) are also faster. 40-60 minutes to do a 120GiB delta with
http took 25 minutes.
I can probably gain more by moving the replication to a 10G network - or at least XOA will be able to deal better with concurrent replication jobs between different servers since it all funnels through XOA.