Afaik, USDX doesn't support VAAPI (video acceleration on GPU instead of CPU). This causes performance issues on low-end machines when videos are running.
A quick test on my Linux box, equipped with a E8400 and AMD HD5770 graphic cards shows that 98% of the CPU is being used for the game when playing with 6 simultaneous players on two screen, and a 720p video is showing in the background. Comparatively, decoding a 1080p video (one a single display) in XBMC on the same setup only uses 15% of the CPU at most.
IIRC, USDX is based on mplayer, and mplayer can handle VAAPI nowadays. Would it be possible to compile USDX with VAAPI support?