Антонимом at a regular pace

instantly immediately promptly instantaneously forthwith directly now pronto straightaway instanter straightway quickly presently headlong incontinently right plumb bang suddenly abruptly speedily swiftly posthaste rapidly forthright ekdam spontaneously PDQ at once right away straight away without delay right now straight off in a flash this minute this instant post-haste right off on the double on the spot in an instant in a trice in no time that instant there and then like a shot this very minute then and there tout de suite in a jiffy that minute in two shakes toot sweet like a flash in double quick time in a second in a moment that very minute here and now in a minute double quick all of a sudden in less than no time before you can say Jack Robinson in a wink in the twinkling of an eye before you know it at a stroke on a dime quick as lightning on the instant double-time first off in the blink of an eye like greased lightning at the speed of light in a split second like lightning in a fraction of a second like an arrow from a bow unexpectedly asudden unanticipatedly sharply unawares without notice without warning by surprise all at once out of the blue on spur of moment on the spur of the moment short

at a run at a slant at a slow pace at a snail's pace at a standstill a taste of at a reduced price ataraxy ataraxia at a rate of knots ataractic at a premium