Quark 5 is not OS X "native."
Both Quark 4 and Quark 5 are quite slow in Classic; Quark 4 also tends to exhibit screen display glitches in Classic. As a result, you really want to run Quark when you are booted into OS 9, not in Classic.
Quark 6, which is not yet shipping, is OS X native. (In fact, Quark 6 requires OS X; it doesn't run in 9.)