Send it in. Diary of a Geek Game. Classic runs such tasks close enough to a plain Mac OS 9 system as to be indistinguishable without a stopwatch. The truth is, any classic application will take a small redraw speed hit from running in the classic environment.
Classic applications continue to exist in their cooperative little world, of course.
Classic, more than any other OS X application, has hooks deep into the core OS, all the way down to the Mach kernel. As an apparent consequence, it seems to be the most susceptible to bad behavior in response to system load. The next offender has no such defense. The OS X Finder is the undisputed king of unresponsiveness among the OS X native applications. We've already touched on the horrendous list-view resize problems, but wait, there's more.