Try running the same distro on a live USB drive. If it runs faster on USB then it's likely your HD that's the problem. As many have said, XFCE is very light weight, if you can't get KDE running smoothly. As an experiment you could try installing Fedora. The way the installer works is that it boots a live version, which you can use. In the live version you have an icon to install to your system. Use the live version a while, then install and run the installed version. Normally the installed version should be noticably faster.
Try running the same distro on a live USB drive. If it runs faster on USB then it's likely your HD that's the problem. As many have said, XFCE is very light weight, if you can't get KDE running smoothly. As an experiment you could try installing Fedora. The way the installer works is that it boots a live version, which you can use. In the live version you have an icon to install to your system. Use the live version a while, then install and run the installed version. Normally the installed version should be noticably faster.