Having more devices should make the router run slower.....most of the perceived "speed" comes from the speed of your internet connection. So, having more devices connected using up your available bandwidth from your ISP will make your speed seem slower.
jensen,
How do you know if you only have 1 device connected to the router? Are you using the encryption/ security settings?
Also, your computer might not be able to handle 300mbs, the built in wifi card might max out at 174mbs. Do you do the Microsoft updates? There could be a software update for the wifi that will allow it to take advantage of the router abilities.