Fiberoptics 100/50 Mbps line, during the test dedicated to this one PC alone. Cisco router that can for sure handle the speeds, third party speed tests giving constantly over 90% of promised speeds. Downloads keep constant 12 MBps +- 0.5 Mbps.
Receiving end also Cisco routers, even faster line and a dedicated server only working for this test.
This is clear: cause of overhead is not linespeed or inability to handle it.