You didn't mention the device or devices from which you are testing speeds. Are speed tests from all devices the same? Are they 5GHz capable? If not, then they are communicating with the router on the 2.4GHz band, which is more subject to interference from other sources (cordless phones, amateur radio, etc). You can try changing the wireless channel on either band.
If the router is located near any other electronic devices, move it away. For that matter, you may want to try adjusting the location of the router by as much as you can.
If you haven't already, you should also check for the latest firmware on your PC's wireless adapter.
I have a asus RT-N56U router and I've had bad speeds with it for the past two months. I have cox internet service and plugged directly into my modem I get speeds about 30mbps down and 20mbps up and with the router I get around 15mpbs down 5mbps up. When I test speeds from router I am not far away I am sitting right next to it.
I've tried resetting both the modem and router multiple times, I also upgraded the firmware on the router and restored to default settings. can anyone help?