This sounds like a network problem. Something about your enviornment is forcing the printer to negotiate its pathway to the destination folder. I assume it hits a few timeouts before it is finally allowed to proceed. To get to the bottom of it you are going to have to use some trial and error to isolate the possibilites.
For starters, try rigging up a simple test network and hook up the scanner to it. Think of something like a router, two PCs and the printer only. No internet or anything, just the most basic network possible and test the scan to folder feature.
If the printer is still slow when running through the test then you know the issue is on your printer. It should be replaced under warranty or swapped for a different model. HP should be able to respect this test enviornment scenario and run you through replacement options if still under warranty.
If the printer can perform like normal on your test network then slowly add one component to it that mimics your normal enviornment. Keep adding pieces until you discover where the bottleneck is.
Granted this approach is time consuming and you need to have some knowledge of your network to proceed, but its your best chance to isolate what is actually wrong. You could also try plugging into different Ethernet ports and see if you can isolate the problem that way. Not sure how complex your network is but one segment may be more successful than another.