Solution
EOS-1D X Mark III’s [Movie Servo AF speed] is settable when [Movie Servo AF] is set to [Enable] and [AF method] is set to [Spot AF] or [1-point AF]. Additionally, this function is only enabled when using the following lenses compatible with slow focus transition during movie shooting. (USM lenses released in 2009 or later and STM lenses are compatible with this function.)
EF24mm f/2.8 IS USM
EF28mm f/2.8 IS USM
EF35mm f/1.4L II USM
EF35mm f/2 IS USM
EF40mm f/2.8 STM
EF50mm f/1.8 STM
EF85mm f/1.4L IS USM
EF100mm f/2.8L Macro IS USM
EF300mm f/2.8L IS II USM
EF400mm f/2.8L IS II USM
EF400mm f/2.8L IS III USM
EF400mm f/4 DO IS II USM
EF500mm f/4L IS II USM
EF600mm f/4L IS II USM
EF600mm f/4L IS III USM
EF8-15mm f/4L Fisheye USM
EF11-24mm f/4L USM
EF16-35mm f/2.8L III USM
EF16-35mm f/4L IS USM
EF24-70mm f/2.8L II USM
EF24-70mm f/4L IS USM
EF24-105mm f/3.5-5.6 IS STM
EF24-105mm f/4L IS II USM
EF70-200mm f/2.8L IS II USM
EF70-200mm f/2.8L IS III USM
EF70-200mm f/4L IS II USM
EF70-300mm f/4-5.6 IS II USM
EF70-300mm f/4-5.6L IS USM
EF100-400mm f/4.5-5.6L IS II USM
EF200-400mm f/4L IS USM Extender 1.4x
NOTE