Traditionally, yes. However, "slow" is increasingly used as an adverb, and I am sure you will hear things like "This car goes very slow". I do not recommend you copy this usage though; using "slow" in this way is still generally regarded as an error.
I wouldn't say it's still regarded as an error. It's a recognised meaning in most dictionaries. In some contexts I think it fits better than "slowly", such as telling someone to go slow, meaning "Be cautious/careful". (Yes, the OP was asking about the usual, literal meaning but it is used figuratively too.)