I could do 30 but it'd be completely illegible. Another big part of how quickly you write is what you're writing, obviously - with text response it all comes down to how well you know the text in relation to the question, and with context it's about how easily ideas about the prompt flow within your piece. In this case I reckon it's better to slow down a little bit and develop your ideas as much as possible - concise explanation of highly complex and cogent ideas beats an elaborate discussion of less well thought out arguments any day.
I find myself able to write so much more quickly for Lit and LA because it's just non-stop analysis of presented written material - you don't really have to stop and think to yourself 'what am I going to write next' because you take each sentence/whatever as its given to you, especially if you've had 15 minutes already to sequence your arguments and what you're going to write about when.
Plus, legibility is a big deal - my politics teacher always takes marks off for handwriting, even if you've doubled the word count. Definitely a sacrifice worth making (length for legibility).