What could be reasons for my rsync, which is syncing two remote servers through ssh, to slow down over time like this? It keeps happening. How to check what is the bottleneck?

Hi all, sorry if this has been asked/discussed before (I couldn't find any directly overlapping posts): I have been running the Nextcloud snap now for quite some time, and although things have run quite smoothly, I never really managed to properly back things up. I make weekly backups of the database, config and data, but it's very hard and time consuming to glue these elements back together. And as they say: when you can't check whether a backup works, it's not really a backup. I have been experimenting with KVM/qemu lately and things look pretty great. The idea of simply backing up the entire OS that runs Nextcloud (a backup that you can easily deploy/run somewhere else to test if it's working) sounds very attractive. Reading around, however, tells me that some of you recommend running the Nextcloud docker (instead of a VM). My questions: 1. What would be the advantage of running Nextcloud as a docker, instead of within a VM? 2. What would be a sensible way to have an incremental/differential backup of the VM/Docker? 3. The storage usage of my Nextcloud instance exceeds 1TB. If I run it within a VM, I will have to connect it to a 2TB SSD. Does it make sense to add the external storage space to the VM? How does that affect the ease of backing the full VM up? Or (as I have read here and there) should I simply put the entire VM on the external SSD?