Speed and regression

Hi.

I'm using rdiff-backup 2.2.6 for a large amount of data. All traffic is 
encrypted via stunnel.

I have two concerns:

1. Speed

With SSH, the speed I've seen is 25-50 Mbps, probably averaging 30-40 
Mbps. iperf3 gets around 500 Mbps between my local system (destination) 
and the remote system (source). The SSH speed seems excessively slow to 
me - maybe 6-8% of the measured bandwidth.

I switched to mapping the data source via NFS. Now I'm seeing full speed 
for the transfers.

Why is SSH so incredibly slow? Is that expected? I'm okay using NFS but 
I liked the convenience of SSH.

2. Regression

As noted, my backup was very slow so I stopped it (CTRL-C). The next 
time I ran the backup, it went into "regress" mode, so I lost all the 
progress. Is there no way to avoid this and resume somehow? I couldn't 
find anything in the FAQ.