A bit more than three years ago, I renewed my main machine and bought an Athlon XP 2600+ with 512MB of RAM and a 80GB hard disk. The speed boost I noticed in games, builds and the overall system usage was incredible — I was coming from a Pentium II 233 with 384MB of RAM.
With the change, I was finally able to switch from plain window managers to desktop environments (alternating KDE and GNOME from time to time) and still keep a usable machine. I was also able to play the games of that era at high resolutions. And, what benefited me more, the build times of packages and NetBSD itself were cut by more than a half. For example, it previously needed between 6 to 7 hours to do a full NetBSD release build and, after the switch, it barely took 2. On the pkgsrc side, building some packages was almost instantaneous because the machine processed both the infrastructure and the source builds like crazy.
But time passes and nowadays the machine feels extremely sluggish. And you know that hardware does not degrade like this so it's easy to conclude it's software's fault. (Thank God I've done some upgrades on the hardware, like doubling the memory, replacing the video card and adding a faster hard disk.)
I'm currently running Kubuntu 6.10 and KDE is desperately slow in some situations; of course GNOME has its critical scenarios too. (Well... it is not that slow, but responsiveness is, and that makes a big amount of the final experience.) The problem is they behaved much better in the past yet I, as a desktop user, haven't noticed any great usability improvement that is worth such speed differences. As a side note: I know the developers of both projects try their best to optimize the code — kudos to them! — but this is how I see it in my machine.
Another data point, this time more objective than the previous one. Remember I mentioned NetBSD took less than 2 hours to build? Guess what. It now takes 5 to 6 hours to build a full release; it's as if I went back in time 3 years! Or take pkgsrc: the infrastructure is now very, very slow; in some packages, it takes more time than the program's build itself.
I could continue this rant but... it'd drive nowhere. Please do not take it as something against NetBSD, pkgsrc and KDE in particular. I've taken these three projects to illustrate the issue because they are the ones I can compare to the software I used when I bought the machine. I'm sure all other software suffers from slowdowns.
Anyway, three years seem to be too much for a machine. Sometimes I think developers should be banned fast machines because, usually, they are the ones with the fastest machines. This makes them not notice the slowdowns as much as end users do. Kind of joking.