Sounds like the slide velocity is getting reduced by friction with the new barrel. After you have shot the new barrel a bit you should be able to see wear marks on certain areas of the barrel. Most likely around the front of the bbl and on the locking lug. Those surfaces may require a bit of polishing with 000 steel wool if they are really roughed up. Otherwise they gun will probably start cycling better as you wear down the high friction areas.
The Glock is recoil operated so you may get better results initially by using heavier bullets loaded to max velocity. Also - keep the contact surfaces well oiled. Avoid going to a weaker recoil spring as the slide velocity could get too high once the friction issues are resolved.