Wifi crawling when using a wired pc

So I just hooked my brother up by updating his pc so he can game at my parents house. The problem is when he's gaming on his wired pc the rest of the house can't do much of anything on the wifi. They have the surfboard modem router that they rent from comcast and I've never had any experience with this peice of hardware. 

Any thoughts on how to fix this or what is going on? At my house this doesn't happen but I'm using a linksys router and slightly older modem.