Answer: rapidly

AnswerCrossword Clue
RAPIDLYHow the express travels
rapidlyswift
rapidlythe swift-moving part of a river
rapidlyRAPID, swift (speedy (swift))
RAPIDLYDouble-quick
RAPIDLYAt a fast clip
RAPIDLYWith dispatch
RAPIDLYAt a clip
RAPIDLYChop-chop
RAPIDLYExpertly
RAPIDLYHow the Niagara flows
RAPIDLYWith celerity
RAPIDLYQuickly.
RAPIDLYWith celerity.
RAPIDLYWith dispatch.
RAPIDLYApace
RAPIDLYOn the double