Every channel and every show is encoded at a certain bitrate, and some are 480, some 720 and some 1080. Higher resolution and higher bitrate equals a lot more data to stream.
Whats your network connection from the computer with directv2pc? If its wireless, that might be the culprit. Try putting the computer close to the router and hooking it up with a wire, or run a really long wire if its a PITA to move the computer. If you're using powerline, same potential problem. Wireless and powerline work for some people with the right gear and the right environmental conditions, but the quality of service can be a real problem.
GPU is a lot more important than CPU for current versions of directv2pc, while cpu was more important than gpu for the original versions of the s/w.
I can run the current version of direct2pc on a single core 1.6GHz atom machine with an nvidia ION LE graphics chip. The GPU completely supports hardware decode of mpeg2 and mpeg4, so it works. Its a little slow to start up and get into the playlist, but once you're playing a show its just fine.
So a quad core isnt necessary. You dont even need a dual core. GPU's that will fully do the job arent that expensive either. An ATI 4350 for under $20 will do it fine. Of course if its a notebook you cant do much about that.