Some years ago Apple introduced a feature called “safe sleep”, which dumps the content of the system RAM into an on-disk file. If battery power falls below the threshold that can sustain the content in the system RAM while the Mac is asleep, OSX will use this fallback virtual memory state during the next power-up.
Dumping some GB (2, 4, 8, …?) to disk takes some time, though. If you do not move around your MacBook too often, you can choose to disable this feature for more instant sleep:
sudo pmset hibernatemode 0
You can also remove the perviously used file to regain some GB of storage:
sudo rm /var/vm/sleepimage