My APM in SC1 where about 80 (yes I really am a casual gamer). Now I had realize how much slower I am in SC2 -.-
What are the reasons?
1) Staring at the game.
I find my self very often in a kind of paralyzed situation. Staring at all those these grafical details.
2) Don't know what to do - yet.
I go very fast observer. I anticipate what they are doing - but don't react. This is a thing of knowledge I guess. But often times I just don't know how to counter.
3) Workers find their way.
Pathing the probes to the minerals reduces my focus. I start to watch the game (point 1).
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Don't get me wrong, I love the game =)