Just to add on here, USB for myself with WDR3600, I only get USB 1.1 "Full Speed" throughput on the ath79 platform. On the ar71xx platform, I get USB2/"High Speed" throughput. I have spent some time trying to fix it for ath79, but no progress so far.
The ath79 kernel does give the warning about bus speed, and testing throughput it aligns with what is expected of a Full Speed device.