I'm having the same problem with Transmission 1.7x - 1.83.
Port is opened on router (7000 for NAS and 7001 for Mac) and set in Transmission adequately.
When I seed from NAS (NVX) - i get slow upload speed and its very rear.
When I for test stop NAS and start to seed the same torrent on Mac i starts seeding with max speed just after few seconds with up to 40 connections (that what I observed).
I need to seed to get better ratios.
Any help welcome
Thanks in advance