No, we simply wouldn't understand that. "Arc" is used in technical fields such as geometry (or ballistics) so we'd think you were using some technical jargon. To add to the other great suggestions here, you could also say, "You're slow on the uptake."

I don't think most people will understand that. You could say something like "Not the sharpest tool in the shed." "A few sandwiches short of a picnic." "The lights are on, but nobody is home." but these are perhaps not very nice things to say to someone.... :) Hope that helps!

In the USA, if you tell someone a joke and they don't get it or take too much time to get it, people will say "you are so slow". This is spoken English though :).

No. "Reflex arc" is a technical term in physiology or medicine. We can say "he has slow reflexes," but that always refers to physical responses and it's a plain statement of fact. I like Peachey's "slow on the uptake," that would be very natural in US English. In describing someone who understands a joke late, we can say "he did a double-take." The first "take" is his first understanding of the joke. When he suddenly understands the joke, his expression changes; that is the second "take."

You could also say: "It takes him awhile to get it." "He/She isn't the sharpest." "His response time is a bit long., but he gets it after awhile." :)