I seem to have similar issues. Mind you, I didn't upgrade from v7, I installed a new server and proxy, use the same configuration settings. Backing up the same VM, my Veeam v7 does it in 4 mins at 372 MB/s. My Veeam v8 setup does it in 18 mins, capped at 52 MB/s...
Both backups were started from scratch with no previous restore points.
Bottleneck is showing : source at 99%.
I double checked everything, iSCSI initiators, transport mode, firewall, nothing seems different except from the fact that the new server is physical and has such more resources to itself .
Source is on a HP P2000 G3 10GbE 24 drives SAN , target is a QNAP 1279URP with 12 drives , all connected on 10GbE fabric
Both backup jobs ( v7 and v8 ) have the same source and same target.