I found that my slide didn't slow down all that much. I've got a couple of before and after videos and you can see that although it did slow down it was barely noticeable. It does make a nice clickity clack sound now though. Especially when I rack the slide to chamber a round.
http://media.putfile.com/Airsoft-G18...-Plastic-Slide
http://media.putfile.com/Airsoft-G18C-with-Metal-Slide