[gsg]

It's not all about the quality of the compiler. C compilers have to deal with a language that has terrible semantics for aliasing, and in which cross module information flow is usually limited to what programmers type into header files (although this is slowly beginning to change with LTO and WPO). This is a significant disadvantage for an optimizing compiler. In contrast, JS implementations have the entire program text and can optimise, say, calling conventions however they like.

JS and Lua also have disadvantages, such as their inferior (but memory safe) representation of data, so I would not be surprised if JS were faster for some problems and slower for others.

[justincormack]

That's why Luajit also has an unsafe C compatible representation too, the ffi.