I have a question that I have often wanted to ask of someone that knows something about declarative programming. In my day job, I spend most of my time worrying about trying to make my code run faster (I do low-level user-interface work - building the libraries that application UIs then use, so it's all about moving pixels as fast as possible, getting data to the app as fast as possible etc. If I need to make a SW blit run faster, I go into the function doing the blit, and I may replace some of the C code with some SSE-style vectorised assembler. Or I may choose to run an expensive SQL query once a day, storing the result in a binary file that is then used to feed application data requests, or other things of that nature.

I just don't see how those kinds of optimisations could be done in a declarative language. Do I just have an unusual use-case, or are these languages seriously constrained with respect to the types of programs that they can be used for?