Most of the code efficiency in Vista relates to the graphics sub-system. Programs which are very CPU intensive (like video encoding) will always be slower on Vista
IMO due to the internal checks (which happen many times every second) implemented in the Vista kernel which are used to protect media content from being copied (eg. HD-DVD and Blu-ray in particular). These checks do not occur in XP. Therefore, unless these checks are removed from Vista (very unlikely to happen) then Vista will always be slower than XP for certain tasks.
Video encoding is one such task which is much better suited to XP.