I think it's the wall on the side where the spark plug is that has the most negative effect since it's shrouding the part of the valve where the most flow happens. The air/fuel mixture doesn't flow uniformly around the valve head, most of it flows across it out the side opposite of the intake port. The mixture has momentum and would rather go straight than turn down to go into the cylinder. Big reason raised intake ports are a thing, makes the path into the cylinder more straight.
If you look closely at the pics of the 302 or early closed-chamber 273/318 heads they all have that wall at a much shallower angle and curved away from the valves and the top goes all the way to the edge of the cylinder bore. The more modern chambers (Magnums, aftermarket) have the top of the wall pulled in a bit to make that second small quench area but it's still curved as much as possible so as to not shroud that side of the valves.
I've also heard somewhere before about those one-year-only Ford heads and how they killed power. Not sure what they were thinking it is pretty terrible. Then again IMO Ford has a history of "innovations" that didn't work as intended lol.
