Hello,
I'm really sorry to up this old topic but I exactly have the same problem than Andypc, I got a strong PC configuration (3ghz, 16go ram, arcadevga hd5450 1go, windows 7 64bits...) but Demul is slow. No problem with Taito 2, PSX, Dreamcast... The only way I found is disable auto sort but games bugs a lot... :/
Have you resolve your problem?
Thank you for your answer and sorry for my poor english!