We're also running quite big jobs with VMs >4TB, on a not so slow storage, and I can only confirm what has already been said: IOPS is the key for backup merge and you won't see a big throughput in your stats. Looking at the description of your setup, I can only assume your disks are 7200 RPM SATA in one big RAID5 array: this is more or less the worst setup you can get because:
- SATA 7200 is slow, very slow in terms of IOPS.
- RAID5 should always be created in 4+1 or 8+1 setup or you just add additional IOPS for any write (you have either 7+1 or 6+1+hot spare)
- The additional IOPS on write will impact your reads because the disk is simply saturated and you're reaching system limits
- Also consider that no write cache should be enabled on HDD level in a properly configured RAID system, except if you're ready to corrupt your repository data at first power outage
Veeam is quite smart but nothing comes for free and as already stated you've chosen to trade volume against performances by choosing the forever incremental. You've certainly reached the limits of your disk system in this specific configuration.
PS: I didn't knew the separation of Full and Incremental on different volumes was that beneficial and will definitely look into it, so thanks for the tip