God, I wish I could find it. But there was this one bit I caught through a link that showed the Star Trek enterprise getting blasted AND THEN they put the shields up. It was in reference to this exact same issue, and not only does every SCH go through it, but every job can and will suffer from some kind of delay from what you see, and what is happening inside the server.

As WHM, you have time to react when you all of a sudden see a enemy cast bar filling up. For a SCH trying to shield it, if you react to a cast bar, you're already too late. You will likely already be in the middle a current gcd, plus have to wait the 2.5ish cast time of Galvanize. The effect of Galvanize has a long duration, so the idea is to have it up even before the casting begins. You could say this requires knowing the fight, but you can also say that pretty much all dungeon and raid bosses 50+ pulse AoE like there's no tomorrow. IOW, the boss is very likely to AoE the group before the effect wears off, so just keep it up if you're learning the fight.