Question

is slow adjective ? then How?

Answer 1

Slow also combines with present participles in forming adjectives: slow-burning; slow-moving.In this use it is standard in all varieties of speech and writing. Slowly is by far the more common form of the adverb in writing.

Answer 2

Slow is an adjective because it describes the quality of the speed and expression of motion of the object or the person.
Anything that describes a quality is an adjective.