The ZendGuard offers optimization and encoding for your PHP code. I visited a friend who ownes a license of ZendGuard so that we had the idea to test it with something CPU intesinve like Image_3D . The results were quiet surprising.
I was surprised that simply adding the ZendOptimizer to the php.ini caused a performance breakdown by 5%, and encoding the code, with or without optimization increased the performance loss to nearly 30%. I didn't expect a great performance improvement from a bytecode cache, because the parsing is very minor to all the calculations done while raytracing.
The improvements by APC (enabled optimization) are so minimal, that it can be caused by statistical noise. All the times are the mean value of 5 to 10 iterations - the difference was minimal, so that I didn't see the need to test it more often.
I can understand, that the optimizer does not offer any real improvements, like APC does, but I wouldn't expect such performance impact.
We found similar results when we did Zend/ionCube benchmarking in Feb this year - http://www.olate.co.uk/development/benchmarks.php