You basically have to do all the parsing, but you can delay creating the actual AST and other data structures like byte code, which for us is the really expensive bit.
But if you have source code that you know you will likely be requiring such as the standard library, you can do the initial parsing while compiling the Ruby VM, so you don't end up doing the parse twice at runtime.
Long term what we hope to do is to provide a build of the Ruby VM that includes the version of Rails you are using pre-parsed.
And then longer term we'd like to actually fully parse and initialise Rails (run the top level of the files which are loaded) during compilation, and freeze the heap and store it in the Ruby VM executable. When you run this special Ruby/Rails VM the Rails code is simply mmapped into your address space with all objects initialised and ready to go.
Obviously it'll require some tweaking to delay doing this like starting the web server so that doesn't get run during compile time.
It's a really interesting idea to pre-load the heap with just a set of libraries, which wouldn't be subject to as much change.