"They talk too fast for me."
Translation:Sie sprechen zu schnell für mich.
zu does indeed mean "too" (= excessively) here.
I'd say that the immediate cause for the accusative case mich is that it is right behind the preposition für, and für requires the accusative case.
Prepositions don't require a case for all the following words in the entire sentence, only for the noun phrase that follows them.