A question about RT-AC68U

Hi
I recently buy a RT-AC68U and I am testing it to see what is the maximum speed that I can get from sharing HDD over wireless N and AC receivers.
My laptop is wireless N with one antenna and the maximum speed that I was able to get is 5MB/s and for my cellphone which is AC with I guess one antenna is 6MB/s.
Is it normal?
I was expecting something 35-40 MB/s (of course for 2 antenna AC device) but for one antenna 6MB/s is not low?
Is something wrong with my router?

Thanks