When top rotating, it has an angular momentum I. The faster the spin of the top, the larger angular momentum the top has. Like f=ma. You need torque L to change the angular momentum. The torque applied to the top always is smaller than a constant L(max), that is, the force from the table surface (point to up) vector times r, the tip to its mass center r.

L = f x r = mg * r * sin (beta)
|f| = mg
beta is the angle between table surface and the top's z axis
so L (max) = mg * r
Like f=ma, L = Ia, where a is accelation of the beta

So fast spin top is more stable.

This also shows when beta getting larger, the top is more likly unstable.