Honestly what's your definition of slow?
We like to go the last week of August. Kids are back to school and the parks are light volume. Yeah it's hot, but as long as you are in and out of attractions no big deal.
We also like to go in October way before Halloween., March or May is nice just stay away from the school vacation weeks and the month that gets those lovebugs - they're everywhere. We've gone the first week of December and it's not bad, just stay away from the days they are taping the parade.