Question for @julien-f but you already had an answer from @Danp

Yeah this is relatively normal, it's using 2MiB blocks so there are going to be a lot of requests when you are doing this with large VMs or data. Are these a ton of VMs or a few larger ones? If the later, might be worth moving that data to a NAS of some kinda, TrueNAS or whatever, and have the NAS handle the backup of it so that the VMs remain light and nimble. Of course that only works IF you can do that.