"DOS" in XP is not really the same as in "DOS" in Win 98, which in turn
is not really the same as when "DOS" was "DOS". Try XP's Compatibility
Wizard to tune the settings.
If you really need the performance, consider adding real DOS (or Windows
98) as dual-boot on this machine (or on another machine) to achieve the
performance you expect. Alternatively, you can contact the developers
of this program for advice--maybe they have recompiled it in a way to
work better on XP.