Haste with the current combat system is a major advantage. There are a couple ways to balance this out. First, make all haste spells/abilities only add an additional attack per round. Currently, it adds an additional attack per limb per round.

Next, base the number of attacks from your attack skill %. You get your second attack at 50%, third attack at 150%, fourth attack at 300% and fifth attack at 450%. You could toggle the use of multiple attacks. Also, to provide some more balance, you could add a cumulative 20% penalty. So if you had 150% attack skill, your first attack would be at 150%, 2nd attack at 130% and 3rd attack at 110%.