Decelerated

Comes from the word Deceleration meaning slowing down.
(Decelerated) is used as an insult less specific to certain group of people and focused on the act or behavior of someone instead of the word "Retarded" since retarded meaning is seen as an adjective for disability which is insensitive.

You are decelerated!
What was that decelerated behavior for?
What you just did was decelerated.