How can I make my chicken and cow spawners more efficient?
Well firstly reduce the number of mobs around you. If there's too many mobs already spawned around you, then the spawner will work less well. If the number of mobs around you exceeds the maximum limit, the spawner will not work.
In terms of maximising your loot, it might be best to have you spawners constantly feeding into a mob grinder (I recommend viewing "chicken farm" and "cow farm" by NimsTut on Youtube, he builds great compact ones) and then using the systems he suggests to kill them automatically (if you don't want to use your sword).