How to parse DSL input to high performance express

(EDITED both title and main text and created a spin-off question that arose)

For our application it would be ideal to parse a simple DSL of logical expressions. However the way I'd like to do this is to parse (at runtime) the input text which gives the expressions into some lazily evaluated structure (an expression template) which can then be later used within more performance sensitive code.

Ideally the evaluation is as fast as possible using this technique as it will be used a large number of times with different values substituting into the placeholders each time. I'm not expecting the expression template to be equivalent in performance to say a hardcoded function that models the same function as the given input text string i.e. there is no need to go down a route of actually compiling say, c++, in situ of a running program (I believe other questions cover dynamic library compiling/loading).

My own thoughts reading examples from boost is that I can use boost::spirit to do the parsing of the input text and I'm confident I can develop the grammar I need. However, I'm not sure how I can combine the parser with boost::proto to build an executable expression template. Most examples of spirit that I've seen are merely interpreters or end up building some kind of syntax tree but go no further. Most examples of proto that I've seen assume the DSL is embedded in the host source code and does not need to be initially interpreted from a string. I'm aware that boost::spirit is actually implemented with boost::proto but not sure if this is relevant to the problem or whether that fact will suggest a convenient solution.

To re-iterate, I need to be able to make real the something like following:

const std::string input_text("a && b || c");
// const std::string input_text(get_dsl_string_from_file("expression1.dsl"));

Expression expr(input_text);

while(keep_intensively_processing) {
    ...
    Context context(…);
    // e.g. context.a = false; context.b=false; context.c=true;

    bool result(evaluate(expr, context));
    ...
}

I would really appreciate a minimal example or even just a small kernel that I can build upon that creates an expression from input text which is evaluated later in context.

I don't think this is exactly the same question as posted here: parsing boolean expressions with boost spirit as I'm not convinced this is necessarily the quickest executing way of doing this, even though it looks very clever. In time I'll try to do a benchmark of all answers posted.