That's due to the fact that it is limited by its acceleration. I know this is maybe not the most obvious thing (I plan to change this in the future). When an object is rotating it experiences an acceleration (see https://en.wikipedia.org/wiki/Centripetal_force) and since the acceleration is limited, the rotation speed it limited.
Currently however the acceleration of the AIPath script is not exposed in the inspector (another reason I plan to change things in the future). You will find it around line 213 in the AIPath.cs script
// a = v/t, should probably expose as a variable
float acceleration = maxSpeed / 0.4f;
The value '0.4' is the approximate number of seconds it should take for the AI to reach its top speed. Decrease that value and your AI should turn much faster.