Well, the longer it sits, the more time it has to let the oil drain out of all those places. And if there are things clogging the channels where the oil has to get to upon start up, it will take that much longer to do the job of lubricating. Chunks of gaskets, broken plastic from your old tensioners, etc. could've made their way up into places you cant see, but that's just one possibility.

Try an oil analysis next time you change the oil (Blackstone Labs, etc.) That might tell you something about metal content or unusual levels of contaminants in the oil.

 

I heard an ominous knocking from my engine.
Turned out to be the wheel on the belt tensioner. I confirmed by running the engine without the belt and by stethoscope.

 

Did it sound like this? check the video http://www.gusglikas.com/AutoRepairBeltTensioner.htm