Some time ago I noticed that editing text of a medium sized MXML file had gotten extremely slow in Flash Builder 4. With ‘extremely slow’ I mean a painful lag between key presses: you type (or delete) 3 characters, you see one appearing, wait one second, a second one appears and so on. Unbearable. I had just imported my project from Flex Builder 3 and there were no problems there.
The problem was that under the Project > Properties > Flex Build Path > Source Path tab I had added a few paths to the source code of some libraries I am using, including Flex. I was using this to jump into the code during debugging. Turns out if you are already linking to those libraries, you can add the path to their source code in the Source Attachment disclosure field of that library under the Project > Properties > Flex Build Path > Library Path tab.
Then remove it from the Source Path tab: the text editor will all of a sudden become super fast. As it should be. The less source paths you have under the Source Paths tab the faster it will be. You’ll still be able to jump into the library source with F3 when needed.
An Adobe employee on the Adobe forums seems to imply there may be some issues with doing this, but after a few weeks I have seen none.