Just say "I'm sorry, but do you think you could please slow down a bit?" :) that is really all. The person you are chatting to should not get offended by this!

on the other hand it is much better if you try the faster natural speech. Don't try to get each individual word, but grab a whole clause or sentence and focus on it, not just the words - receive the sentence and focus on it, get it into your memory and accept the whole thing to translate - it can be done, and is much quicker than word by word. When someone speaks to me slowly in Chinese I always grimace and say 'w-o b-u s-h-I n-a-m-e b-e-n! slowly slowly :D 老鬼 Otherwise 'Please, again, a little slower'