たちまち
たちまち · tachimachi · JLPT N2 · adverb
all at once
⚡ 立ち待ち = stand + wait: a wait so short you never sit down, so instantly
Likely from 立ち待ち, 'standing while waiting', meaning a very brief wait. Since you barely stop to sit, the event happens right away, suddenly and all at once.