I want to chip for the issues mentioned here. Feel free to correct me.
USB storage performanceThe router supports the USB 3.0 protocol but that does not mean the CPU is optimised for that. NAS exists for a reason. The recently released RT-AC5300 is hardware accelerated for file transfer and I manage to achieve near to 100MB/s. Check out the test on my site.

Check out the Wireless 2.4Ghz professional menu, there is an option for USB 3.0 interference. You can improve performance with that. USB 3.0 interferes with Wireless 2.4Ghz. Always use Ethernet for a proper benchmark.
Wireless PerformanceTake the RT-AC55UHP for example. Asus advertise is as AC1200. That is combined throughput of all the bands. AC1200 = 867Mbit on 5Ghz + 300Mbit on 2.4Ghz rounded up.
Want to know the actual wireless throughput? Conduct an iPerf3 test. I get 300+Mbit with the RT-AC55UHP on 5Ghz. That is on the best case with good PCIE wireless card and a close distance. Don't expect to get the full claimed throughput especially on 2.4Ghz. Interference, attenuation and noise exist.
My test on RT-AC55UHP
16MB/S write and 20MB/s read on my external HDD. Samba with guest login enabled. Conducted via Ethernet LAN.
20MB = 160Mbit. You need 160Mbit on wireless to achieve this kind of speed.
This post has been edited by blacktubi: Mar 13 2016, 12:43 PM