Although the maximum rating is -10 dBm, you will usually not get good performance until -20 to -30 dBm (depending on the actual hardware). That could actually explain why you get poor performance close to the router.
A simple experiment that I will suggest - move the router away and see if performance actually improves. (I suppose moving the router may be easier than the PC). If you can't move it, try placing a sheet of aluminium foil between the PC and the router to increase the attenuation.
If you want to cable up the WiFi link, you need at least 40 dB, possibly 50 dB attenuators. For comparison, attenuation of coax cable (e.g., RG-316) is about 1-2 dB per metre at 2.4 GHz.