Why is my macbook pro non-retina slow to sleep (15-20 s) despite no sleepimage?
The XLab website says that this 15 sec delay is likely something Apple has added for Mac's that support PowerNap. DarkWake is assumed to be an internal name for PowerNap, and is referenced on a Patent. 

For more, please visit the article on Xlab:

http://www.thexlab.com/faqs/mavericks_post_install.html#Delay_before_sleep