もう少すこしゆっくり話はなしてください
mou sukoshi yukkuri hanashite kudasai
Please speak a little more slowly.

Say this whenever someone is talking too fast (obviously).

Here's a somewhat eccentric YouTube video by a Japanese lecturer that talks about this. Instead of ください, he uses 頂けますか (itadakemasu ka). This is a slightly different, more polite way to say please.