immediately, at once, straight away, right away, instantaneously, suddenly, abruptly, all of a sudden, on the instant, at a stroke, forthwith, then and there, there and then, here and now, this/that (very) minute, this/that instant;

quickly, rapidly, swiftly, speedily, directly, without delay, promptly;

in an instant, in a moment, in a (split) second, in a minute, in a trice, in a fraction of a second, in/like a flash, quick as lightning, like a shot, in a wink, in the blink of an eye, in the twinkling of an eye, in two shakes (of a lamb's tail), in (less than) no time, before you know it, on the double, at the speed of light, like an arrow from a bow;

French tout de suite

informal in a jiffy, pronto, before you can say Jack Robinson, double quick, in double quick time, p.d.q. (pretty damn quick), like (greased) lightning, toot sweet

Indian informal ekdam

archaic straightway, instanter, forthright