The problem with "sufficiently smart compilers" is that they can make reasoning accurately about performance difficult. James Hague has a good post about that, "On Being Sufficiently Smart" (http://prog21.dadgum.com/40.html).

Lua does almost no optimization at compile time* (in part so it can compile data dumps rapidly) but I find it easy to predict the performance trade-offs in various implementation choices. In a roundabout way, it still ends up easy to tune.

Still, usefulness and performance are only indirectly related. I think that was PG's original point.

* Yet oddly it ends up quite a bit faster than Python, and there's also LuaJIT.