That is not really correct. Yes sync speeds matter and yes they are slower. that is 100% true but the thing you have not considered is the duration the flash is 'on' for. Generally the flash in a studio situation is the brightest light source by far. So how long it is lit up for is the effective "shutter speed" my dynalite for example the is only on for 1:3800 of a second and some higher speed flashes run 1:10000 and faster. Flashes under these conditions will stop motion.