We were there the last week in April this year (just over 2 weeks ago) and it was crazy busy (there's even a post here on the boards about how unexpectedly nuts it was). In talking with my coworker, who visited during the "traditional" spring break time, mid-end of March this year, it was more crowded when we were there in April. Now.. I don't know if there were actually more people, or if it just felt more crowded for us. Compared to when they were there in March and us during that last week in April... we had reduced (non high season) parks hours and lots of rides closed for refurbishment. I think this most definitely played a role in crowd levels and wait times on the rides that were open.
I'd planned our trip way in advance (9-10 months), before realizing there weren't many quiet times anymore. As I had searched for a week that, in the past, was known to be more quiet. Well... that didn't happen! lol. For future trips, I will definitely consider going at a traditionally busier time.. if it means all (most) the rides are operating and the park hours are longer. I'm sure that's a main reason why it felt so, so busy during our week in April (which was 6 weeks after our spring break and 4 weeks after Easter this year). It was also apparently a break week for many schools/districts on the east coast, food and wine was going on, and we also noticed lots of school groups (dance teams).