You could, and people do, write screenplay in Word or whatever. Trouble arises later when you need to go through revisions and work with other people that work in those production-oriented apps. For that reason alone it's better to do it all in a specialised app.
Not to mention that lots of popular IDEs are written in unnecessarily wasteful/slow environments (Eclipse and Idea which use Java, etc).
As for implementation language, I'm not sure java is significantly slower in the case of the project size we are talking about here.