This entire line of thinking is backwards. Disney CAN build quickly but they don't have to, so they don't. Universal isn't building quickly because they can, they're building quickly because they have to. People forget that Universal was in dangerous shape from a business perspective before Potter. The fire under their *** is from desparation (not so much any more, but when expansion started 5-10 years ago).
Put simply, crowds are going to Universal to experience the latest and greatest attraction. Crowds are going to Disney to experience classic attractions regardless of what year the last E-ticket was built. We can debate the implications of those facts, but facts they are.
Yes, Universal is moving faster than Disney but they're still miles behind in the metrics that matter: room nights and clicks. And they always will be just in the nature of the two properties.