You've expressed a couple of misconceptions there.
An electronic flash is, practically speaking for most purposes, instantaneous.
First, the sync speed is dependent on the camera, not the flash (excepting certain proprietary small flash units that operate on a different principle). It's a matter of the highest speed at which the shutter of that camera exposes the entire sensor/film area at once to capture that (practically) instantaneous flash over the entire image.
For older 35mm film cameras with focal plane shutters, that speed was as low as 1/30 second (although most were 1/60 second). At higher shutter speeds, the focal plane shutter exposes the film/sensor as a slit moving across the film/sensor surface, exposing it piecemeal.
Larger format cameras with focal plane shutters had quite long sync speeds because the shutter curtains had to travel so much farther across the film. Smaller sensors allow for higher speeds simply because the shutter curtains have less distance to travel.
Cameras with leaf shutters expose the film/sensor all at once at any speed.
'Way back in the day of incandescent (foil-filled) flash bulbs, there were bulbs designed to burn a quite long time (as long as a 1/30 of a second), burning as long as it took a focal plane shutter to race across the film. Canon and Nikon have some relatively new small electronic flash units that do something similar, by "stuttering" the flash for a longer period of time.
Because the electronic flash commits all its exposure without regard to the actual speed of the shutter, the shutter speed has no role in controlling the exposure (as long as you use a speed long enough to expose the entire sensor/film when the flash makes its pulse). The only camera setting that matters for electronic flash exposure is the aperture. If you want to use a wider aperture, you must attenuate the flash power in some way--reduce the flash power setting, move the flash farther from the subject, place neutral density filtration in front of the flash, or bounce the flash off other surfaces. Most of these do, of course, change the quality of the light as well.
TANSTAAFL rules photography. There Ain't No Such Thing As A Free Lunch.
--
RDKirk
'TANSTAAFL: The only unbreakable rule in photography.'
