I have a love hate relationship with Eclipse and the ADT.
Mostly it's a hate relationship. I'm not going to go into that now (and yes I know I really need to try
To the point. I found myself really wanting to debug through the Android source. To do this, when I was sitting in one of Eclipse's nice psuedo-code-from-class views, I hit the find source code button and added the root of Android source. This actually worked.
The downfall is that ever since then Eclipse has been non-stop scanning and rescanning this "external folder". The performance has gone to heck and my patience went along with it. So if you happen to have fallen into this predicament and would like to undo the setting and haven't yet figured out exactly where to look, here is my prescription:
Select the Project | Preferences menu.
Select Java Build Path
See that Source attachment is set to your source directory. Select it and hit the Remove button.
wait patiently. breathe.