You can switch this preview to wireframe mode in the system / Performance options and then uncheck the "Use shaded preview" box, now I'm not sure that it changes much on the calculation !!
If I go into my sketch and move a circle (see image). The extrusion preview moves with it and recalculates itself in real time, it's hellish... It didn't do that before... It's an option to uncheck but I don't see where...