Backup to Backblaze stopped A couple things you can try:
1. Try increasing the number of threads - maybe to 7. While I agree that 20Mbit is low if your upstream bandwidth is higher and available, you may have more luck running some additional streams. I assume you have at least 8 CPU cores. If you have more, you could try 8 threads as a test.
2. Try using a larger chunk size. 10MB Chunk Size may be too low as it will create a large number of chunks - and that requires additional I/O. Maybe try 50 MB and see how that goes.
I would try one test at a time.