Xerath Inconsistent Ult

Not sure if this is a bug or due to old limitations of the system, but it feels like whatever code is in place to limit the frequency that Xerath can fire his ult is causing problems. i noticed that some times when i ult i can almost instantly fire, then other times there is a delay. tested as shown in the video above - i'm hitting R repeatedly at roughly the same rate and as you can see there is a HUGE difference in the rate at which he can fire.