My friend says I speak slow. I say:
I could/can speak fast.
can I say could to make it more polite?
It's up to you. Both are correct.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
I can speak fast(er). This is a claim about your ability to speak faster (than you have been speaking).
I could speak fast(er). This is an offer to speak faster (if requested to do so).
I don't see politeness as a factor in this contrast.
CJ