If a speaker is known for her brevity then her speeches must be

Her speeches must be brief (short, concise).
Brevity is a noun that means concise, to the point and shortness of time. It comes from the Latin root "brevis", which is also the root of the word brief, the adjective correspondent to the noun brevity, which also means a concise summary or statement or (something) of a short duration. So concluding, if a speaker is known for her brevity then her speeches must be brief, that is, short or concise.