I have very quiet, fast speech, too. Part of the problem is that I talk to myself a lot in my room, as I write up essays and journals and other meditations. I need to be quiet in that context so that I don't disturb my apartment neighbors... but then that quietness is still there when I address someone in public. It helps to consider the acoustics of the environment you're in. Different environments have different noise levels, and you have to adapt your vocal technique, depending on that. It also helps to lower the register you're talking in; lowering the tone of your voice, and watching the accenting of each word may force your voice into a more typical accent for your region...and it'll make it easier for people to understand you the first time.