What’s on the other side? That Xeon should be able to push at least a few tens of megabytes/s with our encryption and so on - it does for me at least with similar CPUs. So remaining bottlenecks could be the device on the other side, or possibly some network rate limiting that affects us but not btsync (TCP vs. UDP for example).
(You need two returns, i.e. a blank line, to get a paragraph break here but your question is perfectly legible as is.)