i don't know about django or rails, so this is a bit off-topic.
* with plain php, the request goes to apache, then to mod_php. mod_php loads the helloworld.php script from disk, parses & tokenizes it, compiles it to bytecode, then interprets the bytecode, passes the output back to apache, apache serves it to the user.
* with php and an optimizer the first run is about the same as with plain php, but the compiled source code is stored in ram. then, for the second request: goes to apache, apache to mod_php, apc loads bytecode from ram, interprets it, passes it back to apache, back to the user.
* with hiphop there is no apache, but hiphop itself and there's no interpreter, so request goes directly to hiphop and back to the user. so yes, it's faster, because of several reasons:
* faster startup because there's no bytecode compilation needed - the program is already in machine-readable code. so no per-request compilation and no source file reading.
* no interpreter. machine code is not necessarily faster - that depends on the quality of source translation (hiphop) and the quality of the static compiler (g++). hiphop translated code is not fast compared to hand-written c code, because there's a bit of overhead because of type handling and such.
* with node.js, there's also no apache. the script is started and directly compiled to machine code (because the V8 compiler does that), so it's kind of AOT (ahead of time) compiling (or is it still called JIT? i don't really know). every request is then directly handled by the already compiled machine code; so node.js is actually very comparable to hiphop. i assume hiphop to be multithreaded or something like this, while node does evented IO.
facebook claims a 50% speed gain, which is not really that much; if you compare the results of the language shootout, you'll see for the execution speed of assorted algorithms, *php is 5 to 250 times slower*.
so why only 50%? because ...
* web apps depend on much more than just execution speed, e.g. IO
* php's type system prevents hiphop to make the best use of c++'s static types
* in practice, a lot of php is already C, because most of the functionality is either built in or comes from extensions. extensions are programmed in C and statically compiled.
i'm not sure if there was a *huge* performance gain for hello world, because hello world, even with a good framework, is still so small execution speed could be negligible in comparison to all the other overhead (network latency and stuff).
imo: if you want speed *and* ease of use, go for node.js :)