The power of luaJIT is that isn't doesn't take much work to get there. If you do normal things it ends up being very fast by default.

Yeah, no, I agree. But my experience with it has also been that as soon as you start writing anything except simple code, its performance starts to suffer, while Chez handles complex abstractions more gracefully. So I suppose comparing them directly might be a bit unfair both ways.

What kind of things end up being slow in LuaJIT?

Can't claim I remember much detail given how long I haven't worked with it. IIRC one of the issues had to do with tossing around anonymous functions inevitably causing it to give up on JITing and drop out into the interpreter