This sort of snuck up one me so its hard to say when exactly it started but Avast has started having a real impact on the time it takes to open files and load webpages. Its particularly noticeable when loading fraps, it can be a good 10 seconds after clicking the icon before the administrator prompt turns up.
For the sake of experimentation I've uninstalled Avast for the moment and installed MSE (because of its small footprint and easy uninstall) and my computer is once again as snappy as it should be. Its great how Avast have been improving lately but you really shouldn't lose sight of the fact that one of Avasts biggest attractions is its speed.