There are many synonyms for "speak fast," including chatter, gabble, prattle, jabber, blabber, and yap. Each word implies a certain level of speed and perhaps even an element of mindless or excessive talking. "Chatter" suggests quick, light conversation, whereas "gabble" implies rushed and muddled speech. "Prattle" indicates idle chatter or babbling, while "jabber" is often used to describe unintelligible or nonsensical speech. "Blabber" suggests a lack of thought or intention behind the words being spoken, and "yap" can refer to annoying or repetitive speech. Overall, there are many different words with similar meanings to "speak fast," each carrying its own connotations and nuances.