This rhetorical question is asked sarcastically and means that they should be doing the opposite. This question can be written as a statement. Alternate translation: "You should be acting now!" (See: [[rc://en/ta/man/translate/figs-rquestion]] and [[rc://en/ta/man/translate/figs-doublenegatives]])
These two negative words "not" and "slow" together emphasize the positive idea to attack quickly. Alternate translation: "Hurry! Attack" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])