Discuss and learn French: French vocabulary, French grammar, French culture etc.
"rapide" is an adjective . You can use "vite" to say "quickly" .
"rapidement" is also used but "vite" is more common , I think.
Je dirais :
"Vous parlez français trop vite. Je n'ai pas le temps de tout comprendre".
"You are speaking french too fast. I don't have time to understand it all".
another advice :
When French people talk together, they speak fast and they very often speak wrong.
It is a good thing to ask them to slow down but also to enunciate, it is not always the speed that makes a French conversation difficult to understand for a student but the many contractions or "image words"/"Argo"/"bad words" we use without paying attention.
you can say :
"Vous parlez trop vite, je ne comprends pas" (You speak to fast, I don't understand)
"Pouvez-vous parler moins vite et articuler s'il vous plait ?" (Can you slow and enunciate please ?)
Don't be afraid to be rude! It's perfectly fine !
In my point of view, speaking a fast french in front of you who try to grasp words is not very courteous ...
Just my opinion of course ^^