Tommy,
Some controls (like ThemedControls) create a lot of images to improve the visual style. So, if performance is required, you'll need to avoid it on older machines.
To satisfy all your customers, you'll need to create settings on your app to let the user choose what he wants, but note that in some cases it will require the creation of different controls (native in one case and the themed in other), and this is really impossible to manage.
Maybe the best solution for you is to use controls that can be skinned or not based on an user choice, but I don't know if such controls exists.
PS: I'm assuming you will not use native VFP controls, since they are linked to the OS visual style.