I am also a slow writer who can't write as much by hand, but I managed to get a 4 for the writing section. To be honest, the writing section isn't nearly as important as the other science sections and your overall score. Sure, you don't want a 1 or a 2, but that would hold as much weight as would a 60 or a 70 composite. So, you can have a relatively small essay that works as long as you can sufficiently answer the prompt.