After establishing these foundations, users can tweak KDE to perform well on older hardware. Several options are listed at the KDE wiki.
Update to a recent version of KDE. The updating game gets old real fast, but generally, with KDE a recent version means improved performance.
Disable wallpaper. Select a pleasing and comfortable desktop screen color. (Configure Desktop.)
Disable background gradients. (Configure Desktop.)
Disable shadowed fonts. (Configure Desktop, Advanced Options.)
Disable desktop icon tool tips. (Configure Desktop, Behavior.)
Minimize the number of device icons on the desktop. (Configure Desktop, Behavior.)
Virtual desktops. Limit this to two desktops, possibly only one. Consider that most people are not multi-taskers. Typically most people concurrently run only two to three desktop programs. Having a bunch of programs running in standby, which is how virtual desktops basically work, means a lot of extra overhead that most typical users do not need. Although useful, many people get by comfortably with only one desktop. (Configure Desktop, Multiple Desktops.)
Use a plain blank screen saver. (Configure Desktop, Screen Saver.)
Disable the mouse cursor launch feedback. (Appearance and Themes, Launch Behavior.)
Disable themes. Yes, themes add a degree of personalization, but with older hardware, at the sacrifice of speed and response.
Consider using the KDE Classic widget set and KDE 2 window decorations. Yes, this ends up looking like MS Windows 95, but this desktop is easy on system resources. (Appearance and Themes, Style; Appearance and Themes, Window Decorations.)
Disable displaying window content when moving or resizing windows. (Desktop, Windows Behavior.)
Disable shading animation and hovering. (Desktop, Window Behavior.)
Disable transparent and translucent menus and panels. These bells and whistles are CPU and memory intensive. (Desktop, Window Behavior.)
Disable GUI effects, such as cascading menus and drop shadows.
Mouse cursor. A simple non-themed mouse pointer is sufficient for most people.
System sounds. Reduce the number of system sounds being used and use sound files that are small in size and load quickly.
Enable Konqueror preloading. Konqueror will load faster and Konqueror is too useful as a file manager not to have this option enabled. (Control Center→KDE Components→KDE Performance)
Disable as many KDE specific services as practical. (Control Center→KDE Components→Service Manager)
Hopefully these suggestions will noticeably improve your KDE performance with your older box.