I've also taken the MGMT of the esxi to the 4Cables in LACP hoping to get more speed.
I still suspect that network is the bottleneck. Please check the utilization of all four links during the backup. If any of them are underutilized then take a look at this thread regarding link aggregation and try the approach that Nikita has mentioned there.
I've been running into an issue more lately concerning the length of time it takes to run a full backup. We run VEEAM 8.0. This is a fairly large backup; fulls run at around 1.6TB and today when i came in this morning I still saw the the job had been still running, when typically it should have been complete. Right now the job has been running for over 21 hours. Its been very slow progress and will most likely complete successfully. Is there any reason or known problems as to why this may be happening?