Solution
EOS-1D X MarkII’s [Movie Servo AF speed] is settable when [Movie Servo AF] is set to [Enable] and [AF method] is set to [FlexiZone - Single]. 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
EF40mm f/2.8 STM
EF35mm f/1.4L II USM
EF35mm f/2 IS USM
EF50mm f/1.8 STM
EF100mm f/2.8L Macro IS USM
EF300mm f/2.8L IS II USM
EF400mm f/2.8L IS II USM
EF400mm f/4 DO IS II USM
EF500mm f/4L IS II USM
EF600mm f/4L IS II USM
EF8-15mm f/4L Fisheye USM
EF11-24mm F4L USM
EF24-105mm f/3.5-5.6 IS STM
EF16-35mm f/4L IS USM
EF24-70mm f/2.8L II USM
EF24-70mm f/4L IS USM
EF70-200mm f/2.8L 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
As of February 1st, 2016.