Seema has a good post explaining why software rendering is sometimes faster than hardware rendering for layered windows. In short, your software vs. hardware performance will depend on your specific video card. Seema suggests:
Key takeaway: a semi-transparent window is a particularly complex feature for WPF’s rendering system. Figure out early what type of hardware that you expect your app to run on, the performance that is acceptable for your users, and whether you want to use Window transparency.