Does anyone know why....?

I have an arc'd shaft and am using a .5 inch ball nose to rough it, just one path, no step downs.

And I make a finish path using a .125 inch ball nose same function, same everything.

The .5 inch takes 30 minutes to generate and the .125 takes about 1 minute. Does anyone have a clue why this is happening? All the settings are the same. The ONLY difference is the size of the bit.

Thanks in advance.