If you see the little spinning circle in the middle of the video, it's a network problem. If the video is jerky and stops and starts, it's probably a problem with your computer.
In the first case, the data for the video isn't getting from YouTube to your computer fast enough over the Internet. This could be because you have a slow Internet subscription, or because something between you and YouTube is running slow or is overloaded, or because YouTube itself is overloaded. There isn't much that you can do about this, apart from getting a faster Internet connection (but that won't help if the problem is somewhere else on the Net).
In the second case, a more powerful computer can help.
YouTube doesn't use Java, so that doesn't matter. If it's a network speed problem (spinning circle on the screen), a faster computer will not help, and if it's a computer problem (jerky motion), a faster network will not help.
It sounds like you have a nice, fast computer, so the problem is probably in the network.