You're speaking a bit too fast for me to follow. Could you slow down a little?

Literal

Way-of-speaking [subject-が] a-little too-fast [and so-て] me [for-には] don't-understand. A-little-more slowly speak [could you-ていただけませんか].

Two sentences as a pair: the problem (~すぎて, 'too X and so...') then a very polite request. ~ていただけませんか stacks humble ていただく + potential + negative question, maximizing politeness. Perfect for asking a stranger or superior to accommodate you.