If I recall correctly, anything flashing faster than 60 times per second, your not going to perceive as flashing. Also, xenon flash tubes are powered by a charged capacitor which dumps all at once.
Take a picture of a crt monitor at 1/30 of a second and in most cases you'll see the entire screen. 1/60 of a second will show the raster lines of the refresh rate.
The slr flash units that have variable frequency flashes have limited power equal to the overall capacitor capacity divided by the frequency for time duration.