I’m seeing about 5-6MB/s on a 40MB/s connection. I thought this was an issue with B2 (using B2 backend) so I tried using iDrive S3 and seeing exact same speed making me think there is something else going on. I tried changing the read-concurrency and b2 connections and saw no improvement.
If I set the S3 connections and read concurrency to a crazy number like 50 I can push almost 7MB/s but that’s the best I can do.
Does it make a difference when you upload a single big file? Also just to make sure, do you have a 40 Megabit/s or Megabyte/s uplink? Because if you have a 40Mbit/s uplink this sounds really reasonable to push ~ 7 Megabyte per second (as sometimes the uplink can be faster).
@marky just checking in with you to see if your issue is resolved 
I’ve been testing other backends to see how the performance compares. Raising the connections and concurrency really high helped a bit.