KISS means "Keep It Simple, Stupid", a basic rule when running logistics enterprise (which is what I do) should be applied to developement of Linux as well. My linux box which is 733mhz Intel PII has been slowing down over time when applying new pagages and building new kernels. Optimization should be applied.
I'm no coder, but I'd like to see Linux, especially its usable desktops (KDE + Gnome) being much quicker. KDE is widely known for its sluggishness, even on new machines. Something should be done. I doubt that linux community wants to run a Vista kind-of resource hog on their machines.
Apple has done magnifient job with OSX, with each version being snappier than previous, from un-usability of 10.0 to incredibility of 10.4. And all that development over just 5 years.
If several hundred programmers in Cupertino can do the trick of code optimization, then what about thousands of hobby coders across globe?

KISS is a known rule in software development, too. May be you have heard about "A Plea for Lean Software" by famous Niklaus Wirth.

Anyway I too agree that Linux GUIs need optimization. I don't like seeing my favourite software become bloated. GNOME and KDE are real desktop environments, so I wish optimization would come to them. E17 (Enlightenment DR17) runs fast even with eye candy.

It's easy to optimize software for specific hardware, such as Apple does. It's harder when your software needs to be compiled to run on a wide variety of hardware configurations.

If you really want your software optimized specifically for your box, compile it yourself.

As for KDE and GNOME, they are pretty bloated. KDE has made some improvements in the last couple of releases, but still bloated just the same. You can lighten it some by turning off the eye candy. It's a trade off. Eye candy special effects takes resources.

If your new kernels are slowing down your system, they're probably too big. You can make them smaller by using modules instead of compiling too much support into the kernel itself.

Choosing the correct window manager or desktop environment for your system's specs, and proper configuration is the key. That's the beauty of Linux, you have control.

I find Slackware (properly configured) to run far faster than any other OS on this old machine, and it flies on modern machines with fast processors and loads of RAM.