That is interesting. I remember that when I was using crashplan, I could see a lot of traffic generated from my PC that was not identified to any application by any of the windows monitoring tools I have tried.
After a while, I manage to discover that the traffic was generated by the "Teredo Tunneling Pseudo-Interface". That is a IPv6 tunneling interface that I have never configured, and I believe was enabled by the crashplan client.
So I did a test, and disabled the Teredo interface and my backup speeds dropped, just like in your case.
Would be interesting to see if there is an issue with Teredo interface in 1809 that causes that speed drop.
The Teredo thing sounds familiar, but on my current 1803 install there is no such interface.
Just checked my laptop which is running 1809 and it's not present on that either - although my laptop doesn't have the CrashPlan app installed.
A good test would be to add my laptop to CrashPlan and see if it has the same slow speed, but I don't want to pay for a second CrashPlan device license just for testing.