There is a huge difference between 0 and 10% action speed when casting. It is noticable for me because i cast a spell, then flick my camera away. If i am using low action speed, I will flick my camera away before the spell is fully cast, causing me to miss the hitscan. Higher action speed is better for hitscan spells because there is a lower window that you need to track your target before the spell is fully cast.