MacBook Pro Retina takes a few seconds to power on display after sleep

Maybe the excerpt below can explain.

The full article can be found on OS X Daily, Is Your Mac Slow to Wake from Sleep? Try this pmset Workaround:

If your MacBook Pro or MacBook Air feels slow to wake from sleep after it’s been sleeping for a while, there may be a fairly simple cause: standby mode. Standby mode allows a Mac to potentially have up to 30 days of ‘standby’ time, meaning it can sit in a prolonged sleep state for that long before draining the battery. Basically, standby (and sleep) works by dumping everything out of active RAM into a sleep image file on the hard drive, and then when the Mac wakes from sleep that sleep image file is copied back from the hard drive to RAM. You may have guessed this already, but the reason some Macs take a long time to wake from sleep is that process of copying contents of sleepimage back into memory, and typically the more RAM you have in a Mac the slower the process can be. As you can imagine, it can take a while to copy 8GB or 16GB of data anywhere, even when a Mac has a super fast SSD drive it’s reading from.