Bits and bytes
Internet and network speeds are given in bits per second (Mbit/s), file sizes in bytes. One byte is 8 bits, so a 100 Mbit/s connection moves at most 12.5 MB per second. In practice less: protocol overhead, Wi-Fi, many small files, slow disks or a busy NAS. 70–90 % of the nominal speed is a realistic expectation for large files on a cable.
For a home backup: upload speed is usually much lower than download, so the first full backup to the cloud can take days or weeks. The trick is to do the first copy locally (an external disk or a NAS next to the PC) and send only the changes over the Internet later – see Constructing a Backup.