The same here. Synology DS918+ with the latest IDrive package installed.
The upload speed is 100 kb/s.
A speedtest at speedtest.net using "California Internet Solutions, Los Angeles, CA" as testserver showed me that I have 63.04 MBit download and 10.78 MBit upload.
I found out that if you split your backup into several backup jobs and start them simultaneously you get 100 kb/s PER job. So this must be bandwidth throttling, because I cannot imagine anything else.
As long as IDrive does not get fixed this problem I can only advise to not subscribe if you have plenty of data to backup.
The only responses from support I can read since 2 year old posts are that the development team is working on improvements.
Since 2 years and no progression?
I'm really upset and if the next supportticket (#ID330227916 for reference) contains no solution I will cancel my account and demand a full refund.
I will post an update if the support had been able to solve my issue.