They made it rather easy to apply mass changes to that respect
Look near the very end of gamedata.ini, you'll find:
;------------------------HORDE MOVEMENT DATA------------------------------
#define NORMAL_EVIL_INFANTRY_HORDE_SPEED 28
#define NORMAL_EVIL_INFANTRY_MEMBER_SPEED 32 ; A little faster so when the formation wheels the unit can catch up.
.
.
. etc
Just change those numbers and there you have it.