I recommend you start 'Activity Monitor' (found in Applications/Utilities') before you let the Mac go to sleep. If an Application shows a high CPU-load it could be the culprit.
In you case it's probably the 'Safe Sleep' function, the equivalent to the Hibernation Mode in Windows, which creates a temp. file on your HD to ensure a correct wake-up - even after a power loss. The file is usually the same size of the RAM and is probably the cause of you problems.
Can you give more details to the RAM- and HD-Size?
if you check your activities as i mentioned before, did you notice any activity in
There should be a file called 'sleepimage', which you can remove. It will be rebuild if needed.
You should check your Power Management Settings in the Terminal via pmset
pmset -g | grep hibernatemode
The default should be "hibernatemode 3", meaning that RAM contents is written to the disk upon sleeping. To test if the reading of the sleepfile is causing your sluggish behaviour you can disable 'safe sleep' with
sudo pmset -a hibernatemode 0
Please try that and post your observations.