Have you made sure that the firmware of the router is up to date? To do this you need to get onto the router admin pages. Firmware update here.or you can do it via the router.
Is your wifi adapter driver on the "slow" pc up to date?
Is it slow if you try another browser?
What are the speeds like on this PC when you connect by ethernet.
Your router is wireless-n and this could be causing the problem with your slow pc. The attached article may explain this to you - particularly look at Fix 3. It may be that you will have to change the type of security on the router connection.Have you set up the wireless connection on the slow pc to take account of the kind of wireless standard your network will support - e.g. mixed mode or b/g instead of just n. Your router manual explains this under wireless basic settings - you can get a copy of this from the link I have posted above.