not too sure about the Zend Encoder
but....
I have a couple friends at a local hosting company here and they were using php accelerator, and they said it was more like php deccelator....
Not only did it tend to slow down their scripts, it made the scripts seem to be more buggy and run haphazardly (now i'm not sure if the second is a coding problem), but not likely
I tried to run a simple script on two machines there
one with and one w/o the accelerator
it had different text functions, arrays, math equations, etc.....
and consistently, the one w/ accelerator was 2-3 seconds slower, and this is a simple script, that only took .15 seconds most of the time with the w/o accelerator
this must be a fluke, but i thought i'd give you my knowledge