Maybe try http://www.mikebuk.dsl.pipex.com/beebem/index.html
(native or windows/wine version) or see http://www.google.com/linux?hl=en&q=bbc ... 20emulator
I've used BeebEm/wine in the past and it worked fine (after all, the BBC was only 1MHz wasn't it?), but haven't used any BBC emulator recently.
P.S. what distro/desktop environment do you use (kde, gnome etc.)? It may be faster running a less demanding window manager (e.g. Fluxbox, XFCE etc.). Also, there may be some unnecessary services running, depending on the setup.