@Richard_W There are multiple things that would limit you upload speed.
1. The Synology box itself, I assume it has a gigabit ethernet port?
2. Traffic shaping on your router
3. Backblaze might be restricting your upload speeds at their end. They only have so much bandwidth in their incoming pipe so likely restrict things at there end to allow as many users as possible to upload at once.
If I was in their shoes I would rather have two customers uploading at 10Mbps at the same time than have one uploading at 100Mbps and 9 other potentially queued.
If you found this post helpful, please give it Kudos. If my answer solves your problem, please click Accept as Solution so others can benefit from it.