Man, that sucks.
I can't think of a reason why it would affect IGP more than discrete, other than the fact that IGP is probably running closer to the edge (in terms of having adequate performance) to begin with. The patch just pushes it over the edge.
Hopefully the hit will be lessened as the patch gets refined. I suspect they're taking an "elephant gun" approach out of the gate, so there's probably room for improvement. Software and drivers may also get re-optimized to avoid operations which are severely slowed down by the patch.
Nostalgia isn't what it used to be.