As others have said, it means there will already be lines for the attractions in Fantasyland and Tomorrowland when the park opens at 9:00 am, but the rest of the park won't be impacted. My general feeling is that there is a small impact, primarily in the two lands that are open for MM/EMH, but the impact on the overall crowds in the parks is pretty small and dissipates pretty quickly after opening. They don't start distributing FPs until the park opens to everyone, so everyone is on even footing for those. Overall, when I go, I don't really worry about EMH/MM unless I'm trying to ride Peter Pan with the shortest possible line.