I fully confirm what @PhoggyBuckeye said more than one year ago. The internet usable bandwidth drops to around 3% of the normal value for downstream and 10% for upstream. Still the backup is slow. I backup a Synology NAS with the iDrive app for Synology.
You have a serious problem if this is happenning for more than one year, you keep advertising a bogus service, and you keep "looking into this" as you always say.
It is not a particular problem or configuration and you know it. So, do not answer if it was.