Menu -> More options -> Uncheck Smartcast Range Indicators.
With this option on, when you press the button it causes range indicators to show up and casts the spell when you depress the button. By disabling this, your spells should cast when you press the button thus getting rid of the delay that you're experiencing.
My smartcast is messed up
With this option on, when you press the button it causes range indicators to show up and casts the spell when you depress the button. By disabling this, your spells should cast when you press the button thus getting rid of the delay that you're experiencing.