Imagine a typical American or Canadian "broadband" consumer internet connection with 10Mbps down and 1Mbps up. If you keep your computer on 8 hours a day, it will take just over 9 months to upload 1TB of data to the cloud at that speed. A multi-TB RAID array would take years to back up. (This is why I think Backblaze et al. can afford to offer "unlimited backup" for a flat monthly fee. The customer's ISP is doing all the limiting for them!)
It was a major PITA when I first started using online backup services while in Canada. (TekSavvy FTW!) Despite paying for one of their faster plans, I had to keep my computer on for a couple of weeks 24/7 to make the initial backup.