I have the same problem using the Synology Idrive client (Version 2.03.34). Thank god I still have my Crashplan subscription on the side (getting 17 Mbps on that).
I don't think a cap on the bandwidth is completely unacceptable - but it's got to be way higher than 100 kbps !!
I have the *same* job running with Idrive and Crashplan right now - Idrive requires 30 Days to complete - Crashplan requires 2.4 days....