I probably overstated... I don't think JITs including LuaJIT do much for string manipulation, but that would mean it's on par with Lua.

Off the top of my head I think of Lua/Python as 10-100x slower than C, but LuaJIT might get you within 2x for some numerical workloads. But it will not get you within 2x for string workloads -- it's still in the 10x or worse category.

Someone reported a slower benchmark here but the maintainer argued that is invalid. But he says all the time is within C functions, which is what I was getting at.

https://www.freelists.org/post/luajit/Luajit-string-concaten...

A couple people mention string performance here, without many details:

https://news.ycombinator.com/item?id=12573981

I think the bottom line is that the semantics of Lua strings are completely different than how you manage strings in C, so there's a limit to how much you can optimize the program, even if you have a lot of time to (which the JIT does not.)

Using the C api instead of the FFI is slower in LuaJIT than Lua and vice-versa. So there are still optimal solutions for each one.