Right, but we know, from experience, that using improper hardware (such as that crappy Realtek you have) is a sure-fire way to end up going nowhere fast.
Quite literally, it's almost guaranteed that your choice of hardware is solely and entirely responsible for the lack of ability to dobandwidth measurements. We know this. We've seen it for years. We've documented it in a 1000 places because, as a whole, the forums have had enough "my Realtek sucks, help me make it better" threads. Us admins have considered simply deleting threads outright, with no warning or feedback, for people that are doing exactly what we've told them not to do and somehow expecting a different result.
So when you ask what the problem is, the answer seems way, way more than obvious to me. You chose bad hardware and now you are paying the price of those choices. You are not the first person to have problems with Realtek (you're probably the 500th or so in this forum alone). You are not the first person to have problems with iperf on Realtek (you again are probably the 500th person or so in this forum alone).
So while you want to discuss this issue and come up with the fix, nobody else is interested. It's not a coincidence that *nobody* else has bothered to post. As soon as you posted your specs everyone with a brain said "yep.. some guy that thinks he can ignore our recommendations and still expect it to work". You are probably on most of the people's ignore lists now too. Why? Because you ignored the advice we wrote to avoid having threads exactly like this one. If you ignored our advice, despite it being plastered all over the place like graffiti at an abandoned warehouse, do you really think we're going to spend time on you?
To be honest, I'm not really sure why I'm even spending my time to discuss this with you. Not because I'm a jerk but because I've got zero patience for people that didn't want to listen to my 500 posts saying "don't do that" and you somehow think you are special and deserve special treatment. Reality check: You are not special. You are not the first person to ask your questions.
Nobody on this forum is paid for posting. We are *all* 100% volunteers. You should understand and respect the fact that we've already given you an answer (don't use that f'in hardware). If that's not good enough, you are welcome to find someone that you can pay to tell you the same thing. Frankly, I consider the free advice to be a better choice. ;)
Good luck in your endeavors.