Why does iperf produce the claimed result in multi-threaded testing?

Question

network hardware

There are 2 old servers and 2 optical cards 10Gb/s. When tested with the iperf utility without keys, it shows somewhere around 1.3-1.8 Gb / s. If you run it with the -P 7 switch, thus creating several threads, it shows the declared result. 6-6.5 Gb/s.
Why 6 and not 8-9, I think the problem is that the old server and rests on the bus.
Win2008R2
RSS enabled, ASUS PEB-10G/57811-1S cards, frame size 1514. RSS Queues Queues 2
In the registry HKLM\SYSTEM\CurrentControlSet\Control\Class\network adapter guide\network adapter number\
changed parameters *RssBaseProcNumber 2 and * MaxRS Processors. 12 for your percent.

Answer 1

Specify a larger TCP window size.

Answer 2

Which slots include network cards? Write your motherboard model.