First thing to try is uninstalling and reinstalling the full software solution for your OS from http://support.hp.com.
If that still does not help then you will have to do some trial and error to isolate the performance/slowness problem:
- Test scanning to multiple document types.
- Test scanning from the document feeder and the flatbed
- Test scanning from USB and over the network
- Test scanning in B/W, greyscale and in color.
- Test the basic copy function
Chances are that there is a driver issue with this printer and Win8. It works but is not as efficient as the WinXP version it was originally developed for. There is most likely some conversion going on behind the scenes so that the win 8 machine can understand what is coming from the printer before displaying it to the user.