I am not quite sure what to do with speed statistic. Does it only affect movement speed because if that is so, then it is useless. Does speed affect your shot rate of fire on guns? If it does then that is good and i might use it.

Speed is how fast you run. It scales fairly slowly, and (I believe) is impeded by increasing weight of what you're carrying, so as you grab more stuff, and heavier armour, you actually slow down.

Pump this and no alien can touch you, so it's well worthwhile.