at once, straight away, now, right now, this/that (very) minute, this/that instant, immediately, instantly, in/like a flash, directly, on the spot, forthwith, without further/more ado, promptly, quickly, without delay, then and there, there and then, here and now, a.s.a.p., as soon as possible, as quickly as possible, with all speed;

North American in short order;

French tout de suite

informal straight off, toot sweet, double quick, in double quick time, p.d.q. (pretty damn quick), pronto, before you can say Jack Robinson, from the word go

North American informal lickety-split

Indian informal ekdam

archaic straight, straightway, instanter, forthright