So you're really testing a just-mentioned Windows 10 machine not on the diagram...cool.
So you're:
running RDP; and
a Parsec session too?
So...this is Parsec traffic or iperf3?
Most importantly, given your issue is with a Windows 10 machine...what does a iperf3 on the Windows 10 machine itself (i.e. the one with the actual problem) show?