Canon EOS-1D X Mark III Advanced User's Manual page 268

Shutter Speed
Available shutter speeds in <
your specified movie recording quality.
Frame Rate
> and <
Normal Movie Recording
–
1/4000–1/8
> mode vary depending on the frame rate of
Shutter Speed (Sec.)
High Frame Rate Movie Recording
268
1/4000–1/125
1/4000–1/100
–