Not so responsive ng-clicks on divs

I am making an on-screen keyboard using a div for each button and ng-click to capture click action. When I use it on my Android phone, clicks events from the buttons don’t seem to fire immediately. If I press two buttons in quick succession (as fast as I would type in a native keyboard), only one of the clicks fire.

I have setup an example project here:

PhoneGap Build:

Github Source for sample project:

At first I thought it was due to the 300ms delay in mobile browsers. But I guess this should be fixed by Ionic already. Do you have any other ideas on why this could happen?