At a fast or rapid speed; quickly and with great progress.
From Middle English and Old French 'a' (at) + 'pace' (step), literally meaning 'at a pace' or 'in steps,' which evolved to mean swiftly.
This word is a survivor—it's been in English for 500+ years and still sounds poetic, which is why Shakespeare used it, but most of us now say 'quickly' instead.
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