According to @Constantin, SMB connection is single-threaded. And he managed to max out a single core, doing 10Gbps transfers with his Xeon D-1537.
According to this, Xeon 6140 has ~1.75X better single-core performance. Which leaves us with maxing out at 17.5Gbps. 2GB/s is 16Gbps. Looks like a close call to me.
I’m not saying that it’s definitely the cause. But it easily could be.
Now, what is more interesting, what is your planned workload scenario exactly? I wonder when 10GBps is not enough for a single connection.