I think "gets dirty fast" sounds strange in English - it should be the adverb "quickly"
Whether or not a person says "gets dirty rapidly" "gets dirty fast" or "gets dirty quickly" differs by region. I hear all of those used regularly.
"Rápido" seems strange here to me, shouldn't it be an adverb like "rapidamente"? I know that I'm probably wrong, but In French and it Italian it should be rapidement/rapidamente.