I doubt a rewrite to D7 with undelphiX will do the trick. I suspect the problem lies elsewhere, like for example drivers. I have tried your game using XP (ati radeon 8500) and it worked great. No slowdowns whatsover. (Well except when a battle was about to happen, but I asume thats normal.)

There was a topic about slowdowns some time ago. Perhaps it might shed some light on your problem as well.