Trouble shooting low rate MCS9 in 802.11ac

[Vu Hai NGUYEN]

>The higher the MCS rate the more symbols are packed up per baud which
>means it is more susceptible to interference and corruption.

>You probably don't have an environment clean enough to pull off MCS8
>and MCS9 and you end up with a lot of retransmissions.

>If you're sure you have a clean environment then try putting your
>devices (or them antenna) at different angles against each other and
>different distances.

What did you mean by clean environment? (There's no other 5Ghz wifi in the room,
no radar or device that emit in 5Ghz frequency? I may have other 2.4Ghz wifi in my room, 
hope that it doesn't create any effects)
I'm using an Omnidirectional antennas of PCTEL so I guess it doesn't change anything 
if I change the angle.
One more thing, my processor is already exhausted (0-1% idle) when I obtain 410Mbps in NSS3 -MCS7,
so if I force it to MCS8 or 9, is it possible that a reverse effects is created? The processor gets a lot of work 
to do and then I end up with retransmissions like you said? 
I hope not cause even in VHT40 or VHT20 my processor is not exhausted and MCS9 is still lower than MCS 8 or 7. 

NGUYEN Vu Hai
Acita-Sodielec
Route de Mayres - B.P. 9
12100 St GEORGES DE LUZENCON
FRANCE