Amazing what different software can achieve on the same hardware. Attempting to play an episode of Star Trek The Next Generation, coded in H.264 640x480 resolution, gave endless stuttering headaches under Xubuntu, but works perfectly in PuppyLinux.
Clearly I need to optimise the Xubuntu install, or perhaps start from the Server alternative install and add stuff until I'm happy with that. PuppyLinux is a bit too different and wonky for my tastes.