What you describe sounds abnormal, but... What processor do you have, at what speed and how many cores?
More folks than not may have modern multi-core machines and not feel the impact of Avast's operation, but surely it does take some power to run it. I imagine there are machines without sufficient resources.
Any software developer has to balance the features they provide to people with newer systems against how efficiently their products run on older ones. Folks with newer systems are probably more apt to spend money on things. Many new products (example, those from Adobe) are starting to shun XP entirely.
-Noel