We were there in 2014 for the last 3 weeks of May (and the first week of June). The crowds were quite manageable EXCEPT Memorial Day weekend (Fri through Monday). Besides being a holiday weekend, Friday was the 24 hour event. So if you'll be arriving on Wednesday May 13....you'll be much happier with the crowd levels than if you arrive on Wednesday May 20. I don't think I've heard that they're doing the 24 hours again, but even without that three day weekends are definitely more crowded than the rest of the month.
We'll be there from May 13 to June 15.....and like last year we know to take an extra bag of patience with us for those 4 days of Memorial Day weekend. We spent most of it in DHS for SWW but even that was more crowded than I liked. Had that been our only time there I'd not be happy.