The main problem with old/ef GoW is that everyone builds exactly the same with little to no variation. And when there's little variation in how people build, there is very little variation in how people attack each other. I thought about this watching a game the other day where braviet was at s9 and someone else was at 11. When 11's peon ran into s9, there's really only two spots braviet's hall can be, "middle" or "below" the gold mine.
Braviet's gold mine was in the middle, with his first farm creating two paths to block someone else from popping his peons below with a farm or tower. 11 blocked this with one wall, forcing braviet's peons to pop out below. Now having played thousands of games, this is something braviet has no doubt encountered countless times. Whereas someone like me hasn't yet figured out what's the optimal move now, braviet and most others have. And with the slow ef speed making execution a non-issue, to combat what's happening is just academic for him.
This is one small example. I watched Viruz play a series vs someone where he got 11 three games in a row. Every game he built exactly the same, rax walling him in, smith behind rax popping grunts out, followed by a chop below the smith a bit later to pop a cat inside. He won every game despite the fact that 95% of the gameplay was exactly the same.
On gow bne there's a little more room for building differently which is a big factor, but I think just as big is the fact that at higher speed, execution comes more into play. Things are a little more chaotic, especially if you're trying to break a dual while maintain things at home. Things get a little messier and that makes games vary much more.
tldr: since everyone on old/ef builds the same, every higher level player has tons of practice knowing what to do in every situation because they've already encountered the exact scenario countless times before and equally importantly, does not need to be fast to do what they know needs to be done.
