The speed difference will depend upon a variety of factors. If your computer is RAM starved under OS X but not under OS 9, it will be slower with OS X. On the other hand, if it is RAM starved under OS 9 it may be faster under OS X because OS X has a better virtual memory subsystem. In all likelyhood, hard disk performance will be better under OS X as well. In all likelyhood graphics will be slower, though I seem to recall some mention of disabling special effects.

Any class of software that has changed dramatically over the past five years (such as web browsers) will be lacking under OS 9. But let's face it, most of the productivity applications out there have not changed dramatically over the span of a decade.