In a relatively short time:
— She finished the assignment in no time.

synoniem: very fast.

Roget 113: instantaneously etc. adj.; in no time, in less than no time; presto, subito^†, instanter, suddenly, at a stroke, like a shot; in a moment etc. n.. in the blink of an eye, in the twinkling of an eye, in a trice; in one's tracks; right away; toute a l'heure [Fr.]; at one jump, in the same breath, per saltum [Lat.], uno saltu [Lat.]; at once, all at once; plump, slap; at one fell swoop;" at the same instant etc. n.; immediately etc. (early) 132; extempore, on the moment, on the spot, on the spur of the moment; no sooner said than done; just then; slap-dash etc. (haste) 684.    ... meer laten zien