Same issue with 2011 iMac (with an SSD). Takes ages, over a minute even. A good bit faster with the power button OK.
I've also noticed it occasionally wakes back up from sleep by itself...i.e. right after put to sleep. Wake for network and bluetooth wake are off.
Clean install of 10.13.6; downloaded from App Store from said iMac (via 10.11.6).
Interesting thing is: I tried Safe Mode, and it woke from sleep instantaneously there, even via keyboard. No affect after booting back to "normal mode", however.
[doublepost=1535588995][/doublepost]Ah, so I figured out what was causing it in my case. Login Items: UAD Meter Launcher. By removing that from login items it's waking from sleep fine (after reboot).
I double checked by reinstalling UAD software (this Login Item is added there by default) and same sleep thing again. Removed from login, reboot, wake from sleep in 2/3 seconds.