A lot of that has to do with:
1. Your network interface card (NIC)
2. Your processor speed
3. Your video card and Video RAM installed
For instance, I have a 2007 e-machine with WIndows 7 - an AMD Sempron processor at 2.1gHz, NVidia network card and "onboard" graphics with only 128mb of Video RAM. On a wireless connection, YouTube videos are very "choppy" in full screen. By comparison, I have a new MacMini with a Corei5 (quad core) processor at 2.5gHz, and Intel graphics with 1024Mb of Video RAM, and Realtek network card. Fullscreen videos play like television on it.