Eclipse needs a good amount of memory and recently, unfortunately, has a tendency to crash. Most of the features can be disabled; the UI can be reconfigured to your liking. Btw, I generally disable the javadoc view and just F3 to the location to see the code and docs together (docs are often wrong anyways).

The problem with mockups is that they haven't touched reality yet: there is always so much detail you don't see at the start.