It will help, but Kde 4 has lot of it built-in and some of it can't be disabled completely.
You also don't have to install another distro to try a different desktop. You can try Xfce out by installing it from within Kubuntu. Xfce has desktop compositing, but it can be disabled with one mouse click. Even with compositing enabled on it, you will notice it to be quicker than Kde 4.