Canon EOS-1DX Instruction Manual page 200

3 Setting the Flash
[Flash sync. speed in Av mode]
: Auto
The flash sync speed is set automatically within a range of 1/250
sec. to 30 sec. to suit the scene's brightness. High-speed sync is
also possible.
: 1/250-1/60 sec. auto
Prevents a slow shutter speed from being set in low-light conditions.
It is effective for preventing subject blur and camera shake.
However, while the subject will be properly exposed with the flash,
the background may come out dark.
: 1/250 sec. (fixed)
The flash-sync speed is fixed at 1/250 sec. This more effectively
prevents subject blur and camera shake than with [1/250-1/60 sec.
auto]. However, in low light, the subject's background will come out
darker than with [1/250-1/60 sec. auto].
If [1/250-1/60 sec. auto] or [1/250 sec. (fixed)] is set, high-speed sync is
not possible in the <f> mode.
200
You can set the flash-sync speed for
flash photography in the aperture-priority
AE (f) mode.