I don't recall the article, but in my experience, having separate partition for Mac OS X and Mac OS 9 doesn't necessarily improve performance, but it does seem to improve convenience, at least on NewWorld Macs with the boot picker, because if X and 9 are on the same partition, the boot picker doesn't register both of them, but rather the one most recently booted, so if you recently booted into 9 and want to go to X (or vice versa), and they're both installed to a single partition, you must use the Startup Disk control panel (or pref pane) to switch. Of course, this is all fuzzy now, since the last time I did any of it was at least 5 or 6 years ago.
Hopefully someone with more information can chime in....
c