I wouldn't think you would want the gathering chains slowed down.  You want to keep the rearward speed of the chains at or slightly faster than your forward travel.  If you are getting a lot of trash through the corn head your rolls are worn or your deck plates are set too tight.  Also, in extremely dry conditions you may get more trash.  If your rolls are worn the stalks won't be taken through until they get too far to the back of the deck plates.  This adds materially to trash intake.  Get the roll diameter where it needs to be (done with a welder or roll replacement) and you should see a big difference.  Mike