Vous parlez français très rapide .

[Crack1]

Today I met 2 Frenchmen. 

They were speaking French very fast.

I told them it was difficult to grasp their conversation.

How do I say that you speak French very fast.

Is the following fine?

Vous parlez français très rapide .

[George Hunt]

"rapide" is an adjective . You can use "vite"  to say "quickly" .

"rapidement" is also used but "vite" is  more common , I think.

[Fourrier]

Bonsoir,

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".

Bonne soirée, 

Robin

[Sandra]

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 ^^