Adobe has never been known for writing fast programs. They usually ignore industry advances in hardware acceleration completely, and to make supporting multiple platforms easier, all their software is written on top of their own development layer, further slowing things down. If they were to write some native code for each platform I'm sure they could improve the speed of their software. Instead, like many other developers (unfortunately) they instead rely on the general purpose hardware advancements of the computer industry - CPU, buses, memory.

That's why Photoshop or Illustrator today, on a screaming fast multi-core machine doesn't feel much faster than it did back in the day on a Pentium 90. With regards to Lightroom, based on the sheer megapixel count of the images now in common use, it would be unfeasible to run on such older hardware, even with a tremendous amount of optimizing.

Scrolling through the grid view and seeing mostly pixelated image representations is a little saddening on a Core2Duo at 2.9GHz with a very fast graphics processor.