。
She'll probably make quick progress in her piano lessons.
Literal
She [topic-は] piano [of-の] lesson [in-で] progress [subject-が] fast [probably-だろう].
けいこ (稽古) is a word that carries cultural weight: it specifically means structured practice or training, especially in traditional Japanese arts — 茶道 (tea ceremony), 華道 (flower arrangement), 能 (Noh), 歌舞伎 (kabuki), and martial arts all use けいこ for their training sessions. Applying it to piano lessons here gives a slightly old-fashioned, disciplined coloring compared to the everyday loanword レッスン. 上達 ('progress, improvement') is a Sino-Japanese noun specifically about skill development; だろう gives a casual, conversational forecast.