I thought you either had blowback or you didn't? Can't remember it being a sympom of shooting quickly? You sure it just doesn't have blowback and it is more noticeable when shooting quickly?
Stick two balls in the feed neck without a hopper on and fire once. See what happens to the second ball. Does it jump up or jump out of the neck? If so you have blowback
Best to check you have it for defo first before people suggest how to fix it