occurring with no delay; "relief was instantaneous"; "instant gratification"
(instantaneousness) immediacy: the quickness of action or occurrence; "the immediacy of their response"; "the instancy of modern communication"
Occurring, arising, or functioning without any delay; happening within an imperceptibly brief period of time
 strikes NDH [B], PO > DS, FO up and both move slightly towards signer. (Like the MINUTE part of LAST~MINUTE.) Obviously the sign FAST will not work with "instantaneous speed", which is a common phrase in physics.