"You are fast."
Wen talking to someone directly like that, you use his/her natural sex. So you won't say είσαι γρήγορο (on its own) to a girl because κορίτσι is neuter, the same way you won't say είσαι γρήγορος to a woman because άνθρωπος(=human) is masculine. In both cases, because you are talking to a female person you use the feminine γρήγορη. The same thing applies if you are talking to animals. But if you say είσαι γρήγορο κορίτσι to a girl, then γρήγορο is an adjective to the word κορίτσι and has to be in neuter form. The same applies to the example I gave above saying to a woman είσαι γρήγορος άνθρωπος=You are a fast human. To conclude: Είσαι γρήγορο is not correct in this case because it is not followed by a noun that is neuter, and you would have to use your speaker's natural sex.