Sometimes, when working the lever VERY slowly the round will bind up and not want to feed and it takes a snap or two of the lever to sort things out. Remember, when the lever is fully thrown the round is more or less just loose in the receiver. But, that said, I can work the actions on all my Marlin levers pretty darn slow and with minimal effort.
Make sure all your screws are tight, specifically the carrier screw and the loading gate screw. They don't have to be torqued down to a million foot-pounds, just snugged up.
Without seeing/feeling the action it is always hard to say. You say it works fine when cycled quickly. Does that mean it feeds smooth or is some force required along with being quick?
Do the rounds show any scraping or scratching?
Does it cycle just fine when single loading through the ejection port? What about if no rounds are used at all and just "dry" cycling?
Does the carrier pop right back down when you start the lever coming back? You can see the tab sticking out of the bottom of the receiver.
Does a new round pop from the tube fully into the receiver?
Just some questions to help us narrow in on some things. It may well be like Matt said and be perfectly normal or it might be one of the things LA or Tomray mentioned. Or it might be something altogether different so more details will help us give more focused input.