c_move(ENTITY* entity,VECTOR* reldist,VECTOR* absdist,var mode)

You see, the second vector argument is the amount of absolute movement. You are looking for relative speed (relative to entity's orientation).

if(key_cuu) c_move (player, vector(50 * time_step, 20, 0.1),nullvector, IGNORE_PASSABLE | GLIDE);