Quote - in memory, because the larger my default runtime becomes the longer it takes Poser to load

Sure it does, as Poser builds the library dropdown... And Poser uses also uses/follows regualar file/folder shortcuts, so it's not too hard to answer why it's slowing down when the library gets bigger... Poser 7 added a timeout for the build action to its preferences, so those not using that dropdown can change that timeout to 0 to have Poser start up with the same speed no matter how big the library gets.