To my small amount of modding knowledge compared to most of the people on here, that sounds like you have an air seal problem.
+1. That's probably the biggest "issue" if not the only one.Low fps on strong spring is known problem with g-spec, it has cylinder the same volume as the full lenght vsr, so the bb is out of the barrel before the piston moves at maximum speed, this can be solved with heavier bbs.