The delay you are seeing may be the time it takes the HD to spin up and load the sleep image file. A badly fragmented HD can slow that process down or as drives age, they sometimes simply take longer to get up to speed. Check System Preferences > Energy Saver and see if "Put hard disks to sleep when possible" is checked, that will contribute to the spin up lag. If it is checked, uncheck it and see if you still have the same 5 to 8 second delay.
Many of us used to defragment our drives regularly but because of improvements in disk managment Apple no longer recommends the practice, but I am not completely convinced that it is never advisable.
