It may well be that there are unique characteristics of the Mac OS/hardware that allowed the Bryce team to provide faster rendering than on the PC. If so I see no reason why the speed of the Mac version should be deliberately held back to the same as the PC just to make me feel better. That would unfairly penalize Mac users. But the fact is that I use a Windows box, not a Mac. I may feel pleased for Mac users but how fast or slow Bryce runs on their machines is largely irrelevant to me or my Bryce usage. That B5 will render slower than B4 does annoy me, unless it is also doing more work which will provide superior results.