Did you get your new power cord free from Microsoft? A lot of performance issues can have a root in low/high power and will effect any electronic device. Check your power in your house if you know how to monitor it.
If not, you must have a congested network connection. Find out if your ISP (internet service provider) is experiencing trouble. If they are popular, they may be slow. You might want to switch providers if you hear of others having slower connections.
Finally, check your disk. My kids don't seem to understand that oil, dirt and moisture effect DVD/CD performance. Keep it clean my friend or it will turn on you!