The issue is a lot more complex than that blanket statement. From a software engineering point, if you can fix/change one module, it's a lot easier than rebuilding the whole executable.
Think of it like an engine in a car. You don't replace the whole engine if it's acting up.....you replace just the part that is stuffed.
This is basically the paradigm used in OOP. Code re-use and maintainability through compartmentalising modules.
However, having said all that, Acad is getting a LOT more complex, which requires more complex code...this comes at a price.
Have you noticed the .exe's in task manager when Acad is running? They have included these 'extra' services/dll's in for the last few releases, including one that is a bootstrapper of sorts. It decreases Acad load time specifically, because they realised it was getting slow.
Try this: Unload wscommcntraccon.arx when Acad is running and take note of your speed for a while after. I've found it's a speed killer...it's the communication centre. Unloading it will get rid of an entry in task manager btw....an executable file of the name WsCommCntr1.exe.
Incidentally, have you ever crashed out of Acad, then fired it up again and it takes essentially forever? This is because it hasn't killed a few running entries in taskmanager. You need to kill these entries, then Acad fires immmediately.
Also, if you're running AcadMap 2006 (don't know whether LDD has this) but it has a new 'skin'...you'll know what I mean as it has a 3d look to the applications title bar etc. that is not the same as previous releases.
If this is the case, try typing in 'MAPSKINMAP'....this will toggle the 'skin'.
I've noticed a significant slow down when the 'skin' is enabled.