So why do they work fine with your custom runways, taxiways and tarmacs, but just not with KFLL?
There are several ways of making custom runways. KLAS is not fully customs, only parts of it are. And KDFW it's entirely a league on its own, since it doesn't have any bit of FS9 code inside.
However, the real problem is caused by using FS9 AI models in FSX and THIS is a big no-no. AI it's the thing that impact fps the most in FSX, so is the use of FS9 code, which means when the two combines (AI using FS9 code), there's no better way to slow down FSX...
Unfortunately, a very popular and free package (WoAI) is still using FS9 models, and only commercial packages use FSX native models (UT2 and MyTraffic 2010), but if you have to upgrade your system to reclaim fps that was lost using a free addon, there it goes your savings...sometimes using free addons has an hidden cost which is difficult to estimate.
Flight1's UT2 features 100% FSX native models, and each one of them is impressively modeled with 8-9 LOD levels each, plus the software has interesting optimization features to allow for a busy airport without having too many AI around, it's a very well done product and it's what we recommend to use in FSX.