jit_expr is a clone of the pure data expr/expr~/fexpr~ objects. It just-in-time compiles its expressions so they should be much more optimized than the original. If all works as designed, they should use less CPU than the equivalent vanilla, non-expr, patching and have a significant CPU advantage over the original expr objects.
I've put the external, compiled for 64-bit Mac-OS and 64-bit Linux, up on deken: in pd, go to help menu, find externals, search for "jit_expr".
After installing the external you should be able to change any of your expr family of objects to just in time compile by loading the library, [declare -lib jit_expr], and then prefixing the object name with "jit/", for example [jit/fexpr~ $x1[0] + $y1[-1]].
I believe they are feature complete with the originals but I'd love to know if there is anything that I'm missing or any bugs that you discover.