I might know the answer. wPrime greatly improves from SSE making a Thoroughbred rip a Thunderbird more than 50% IIRC. But your board might not initialize SSE on Barton (yes, SSE on A-XP was an optional feature that required enabling by BIOS).
You can run WCPUID to check whether SSE is enabled (CPU-Z might only show that it's supported by CPU, not actually working). And is you find WCPUID with plugins, there's a plugin to enable SSE on A-XP.
Try to turn it on and check again.