Bcm6328 USB bottleneck

Hi everyone, I'm using openwrt on bcm6328 zte adsl router, image name A4001N1 , it's basically the same router but the LEDs mapping are different.
On the usb port I'm plugging a usb wifi adapter with rt5572 chipset , I'm using the exact same setup on another router to create a 5ghz link between them.
The problem is that I can't get more than approximately 27 mbit/s with iperf, at this point the cpu load is 100% , (actual wifi link speed is way higher of course),my question is this related to the bcm6328 usb driver in openwrt or the processor of the router is not powerful enough?
If it's the processor, shouldn't traffic go into dma or something like that not directly through the cpu?
Any help would be great

99% chance it is the (a) processor... ( efficient use of read the theory here )

i'm guessing it's multicore?

Well no ,its single core 360MHz

assuming traffic did bypass the cpu... how is usb data "traffic"?

Sorry I didn't understand, please elaborate