Could it be that "0.5x" speed is actually 1x, and you're seeing your algorithm run at 2x sometimes on JDK8+ due to some optimization?

(Not a JVM user, so take with a grain of salt).

Yes, 1X, 0.5X are a bit arbitrary. I mean 1X as the speed I know it can run at. So it could well be that with JDK7 the optimizations could not happen and that the question then is why the optimizations do not always happen.

Making each run longer, 1 minute instead of 5 seconds displays the same performance toggling:

0 - Passes: 72705, count: 78498, Valid: true 1 - Passes: 33140, count: 78498, Valid: true 2 - Passes: 71867, count: 78498, Valid: true 3 - Passes: 33137, count: 78498, Valid: true 4 - Passes: 72264, count: 78498, Valid: true 5 - Passes: 72062, count: 78498, Valid: true 6 - Passes: 33021, count: 78498, Valid: true 7 - Passes: 33053, count: 78498, Valid: true 8 - Passes: 71902, count: 78498, Valid: true 9 - Passes: 33024, count: 78498, Valid: true

(JDK17 in this case)

I see you have a bright future in marketing: the actual facts didn’t change at all, but it now sounds twice as fast!