I wrote a small performance test to compare the performance of using eval vs using new Function.

Function creation: http://jsperf.com/new-function-vs-eval-function-generation
Function call: http://jsperf.com/alternatives-to-argument-callee

eval is 16% slower for creating the function,
but functions generated by new Function are 100% slower.

In emscripten, there is already ccall if the user doesn't want to spend time in the generation of the function.