How to 'slippery up' your a/c

Project Fokker's F70/100, PA's A320 - and by the sounds of it PMDG's 737 series - are fairly efficient through the air especially when descending. Some a/c on the other hand do sow up very quickly when the throttles are idled back, even when descending.Is there a way to make them a little more slippery? I'm assuming it would be in the CFG file. I'm not real clever at this kind of stuff. Just wondering if it could be done to make some a/c a little more realistic.