The weakest CPU I currently have access to is my oldest laptop with a 2.0GHz Core2 T7200. Dragging files from the collection browser to the playlist is smooth as silk on it. Exactly how weak do you want it to be?
>All the plasma widgets which attempt to replicate the functionality of the >builtin features in 1.4 (lyrics, for example) use enormously more >processing power (I'd say a factor of probably a hundred to a thousand) to >provide exactly the same feature.
The lyrics applet does more than in 1.4, it scrolls through the song. But you have a point of course. Just as a Python widget typically drains more resources than one implemented in C++. This does not only affect KDE4, even startup of all GNU/Linux distros were script based until the recent addition of systemd. Optimization comes with time, and KDE in general goes a long way to enable that. Plasma in particular have received numerous performance improvements. I believe the current focus on plumbing in KDE is well balanced. Being too religious about it will slow down development.
Make no mistake, Amarok is a real memory hog. Amarok consumes 400MB here, so I wouldn't mind seeing that number go down. However, with 4-8GB available on my main desktops and 32GB on my workstation, it is no show stopper. With 2GB systems it is a serious concern, with 1GB systems it is a show stopper, but frankly, 1GB is not enough for Gnome these days either.