+0

My friend says I speak slow. I say:

I could/can speak fast.

can I say could to make it more polite?

+1

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

Comments  

It's up to you. Both are correct.

 CalifJim's reply was promoted to an answer.