I used Amazon's Glacier S3 storage for off-line backups using Cloudberry's Desktop client. When I start the backup, it shows good performance and an estimate of several hours. Later the performance drops and the estimate moves to over a day.
I appreciate that the the problem could be on the Amazon end but a check through Google does not report any speed reductions and uploads impacted by location of target and Amazon target.
I believe there are two possible causes for your issue: 1) Sagemcom sync no surf issue: if your system is connected directly to a Sagem DSL modem and if you're on VDSL ask to be moved to an IPTV/Interleaved profile in the Bell Direct forum (unless you really like calling 310-BELL). 2) If not #1 above, iCloudberry Desktop seems to use "multi-threaded" upload. This is similar to how bittorrent opens a lot of connections to speed up block transfers: this could be overloading the NAT table of your modem or router. If you can configure how many connections the client opens try lowering it by 50% and so on and so forth until you find a "sweet spot": better to have less threads uploading and a stable connection than lots of threads and an unreliable connection.