Yeah, I've had the exact same experience.
I've got the same Mac. Wakeups from sleep running El Capitan were fast. As soon as I upgraded to High Sierra I noticed that sleep wakeups took about 20 seconds (which seems like forever). I have not found a solution other than setting the Mac to never sleep.
I also have a fusion drive setup, although I don't see why that's relevant -- the same fusion drive setup with El Capitan woke up from sleep in just a second or so.
It's definitely an annoying problem.