You consider scheduler will raise system requirements to level you will notice this? This is bull. There are just scripts and daemons in Ubuntu which make it feel slower on older hardware. About those features it depends if you have them enabled or not. Karmic has many debbuging options and features enabled (like oops reporting, Perfcounters, AppArmor) which can make it much slower than it would be with such options disabled.
IMHO, I think that two areas are mainly responsible for that performance drop: modern graphic drivers (blobs) are getting more and more complicated and designed over newest hardware.. and the kernel itself (I have modern schedulers in mind).