Macbook Pro mid 2012 wakes up slowly and takes time displaying icon previews. How do I solve this?

My mid-2012 15" Macbook Pro (2.3 GHz Intel Core i7 running OS10.8.5) is slow at waking up from sleep mode. I reset the PRAM and repaired permissions but the problem seems to persist. The display of the icon previews is also a bit sluggish. Have you encountered this problem before?

I should probably add that I have two user accounts on this mac, both administrators. A main one for personal use and a second one for work which was created by migrating from an older mac. Before you suggest me to reinstall completely the OS, I would like to know if there's anything less radical and time-consuming that I could do to make my Mac run more smoothly. Thank you!

-M