All skeletons with the same prefix move at the same speed.

Here's some useful results abot speed, from the FATW archives:
fs_fast_x = 120% * fs_x
fs_semi_fast_x = 110% * fs_x
fs_slow_x = 85% * fs_x
fs_2handed_berserker = fs_x
fs_slingernew= 120% * fs_x
fs_2handed = fs_x
fs_fast_horse = 215% * fs_x
fs_medium_horse = 190% * fs_x
fs_horse = 175% * fs_x
fs_cataphract_horse = 157.5% * fs_x

Movement speed increases linerarly with skeleton scale (so that a unit with *skeleton* scale 1.1 will move 10% faster than the 1.0 scale sleleton one)

Descr_skeleton.txt is not parsed by the game anyway, so don't lose your time there.