From Apple's Java development email list:

2D Java 1.3.1 Graphics tests

HeatBugs, Java 1.3
MacBook 3 framerate (higher is better)
Dual 2.5GHz G5 156 framerate (no kidding)

The original poster's guess was that Java 1.3.1 was being run under Rosetta on the MacBook (an x86 powered creature). The fellow who is likely the most well-informed poster on the Java list replied.

My guess is that 1.3.1's graphics under x86 simply sucks, because there's
little incentive for it not to. For example, I doubt that Apple would even
bother porting any of 1.3.1's hwaccel stuff. There are two later Java
versions that are more likely to be used on x86, so the priority of getting
1.3.1 graphics to a level of mere adequacy on x86 is probably pretty low.
My guess is that 1.3.1's speed on PPC is dependent on its PPC-ness. But
that's just a guess.

Seriously? The hardware acceleration is PPC dependent? That's strange. And at a cost of 156 to 3 frames per second? I suppose that potentially makes sense, and tells us two things I used to write about a decent amount on this blog: 1.) Java 1.3.x on Apple is finally dead, gone just as surely as the Classic environment (so quite Jar Bundling to run on 1.3*!) and 2.) Man, Apple went around their elbow to kludge decent Java performance on the Mac OS. To think people on the Apple list attacked (okay, well, we'll safely say "seriously questioned") those who claimed Java on Apple wasn't quite as hot as Jobs had promised...