You talk about velocity with everybody. Just about every pitcher loses velocity as the game progresses until they learn how to pitch and save something for when they need to get tough outs the 3rd and 4th time through the lineup. Obviously there exceptions, and those exceptions are the guys with freaky stuff. There aren't many.
Giolito has a lot of issues right now, but I think velocity is one of the lesser ones. He clearly doesn't trust his delivery and motion. He's all over the place with his release point, which is making him hang pitches. He has trouble locating pitches in the zone so he ends up throwing a lot of meatballs. When he tries to avoid bats, he misses badly. His issues are mostly mental imo. He's in his own head way too much. He's had too many people messing with his motion and it wouldn't surprise me if he has OCD. He's almost like a golfer that's so obsessed with the mechanics of his swing that he forgets the very basics.