@lbloch depending on you internet speed it could be a good result. Syncing 21GB in 20 hours results in AVERAGE speed of 2,3MBit/s (0,3MB/s). Depending on your internet connection. number and size of the files this could be expected sync speed (from the internet). Maybe your hosting provider applies some throttling?
Especially small files are less efficient to sync. I had similar issue shortly syncing within local LAN was as fast al 10-15 KBit/s because of configured external storage. “clean” Nextcloud running on 4-core Celeron with 8GB RAM can sync 50GB of data within 1-2 hours in LAN at average speed of 30-50MB/s (again given network is not a limit the speed greatly relates to the kind of files). fully saturated 1GBit/s network link can ship around 70GB/s netto - 50GB/s from Nextcloud sounds pretty perfect for me.
bandwidth calculator: Bandwidth Calculator (Data Transfer Calculator) - Calculators.io