720p streaming over internet within same country with <100ms delay

I can think of one possibility (just a shot in the dark since you provided no details about your configuration).

Let's suppose both PCs are using wireless. Well, that can present a problem. You see, wireless is half duplex, meaning only two wireless stations can be communicating at any given time. Any other wireless stations must WAIT until those communications cease. Just like a walkie-talkie, everyone takes turns. The problem is, if you have two wireless PCs working through a wireless router (three stations), your *effective* throughout is halved! Unlike wired connections which are full duplex, every time one PC is communicating w/ the wireless router, the other PC is doing nothing, just idling over its network connection. That kills your throughput. But when you're accessing YouTube from that same PC, you don't have any contention for the wireless router (not if that PC is the only active PC). So that PC gets the full, available wireless bandwidth.

I know, it sounds counter-intuitive, how could local access be slower than remote access. But it is possible under the right conditions. Of course, I have no idea if any of this applies in your case, I'm merely explaining one possibility.