There is no "slow" time at WDW any longer. Due to short staffing/ running less rides to increase lines during "slower" times, or just flat out being inundated with huge crowds ( currently spring break), the idea of "slower" no longer exists. Now, I will say this in response to my own observation. Because I just did it so I know its possible. Get your rear end up early if on property and use EMH in the AM. Over 3 days...EPEMH AM 8AM: Went to Frozen, Did Soarin, Test Track, MS, Land all before1030 and without FPs Saved the FPs for evening at AK.
MKEMH 7AM- 7 Dwarves, Pan, POC, JC, BTMRR, HM all done by 930 then did a few other things till 1045 or so, left, went to the pool, saved FPs for Epcot that evening.
AKEMH 7AM- In the parking lot at 610 AM, on FOP at 705, done by 715, line for FOP was now already 60 minutes, went and did Everest, Chester, Safari and got a great vid of the lioness stalking the male lion and pouncing on him, dinosaur...all done by 945. Break time and saved FPs for MK that evening.
Regardless of crowds, if you are willing to get up early for EMH on property or at a springs resort, you can accomplish a TON with minimal to no waits and still have a great day left to enjoy.