An example that shows that Javascript wasn't designed for performance: it doesn't support integers. They can be used in some Javascript implementations that do

An example that shows that Javascript wasn't designed for performance: it doesn't support integers. They can be used in some Javascript implementations that do smart tricks to convert float arithmetic to integer arithmetic, but it is not reliable. If you were designing a language for performance in the first place, this would be a ridiculous method to get access to integers. Same for dense arrays. Yeah, JS is pretty fast if you know how the particular implementation that you're targeting works, but even then there are many things that just need a lot more performance than JS can offer. On the expressiveness front: Javascript doesn't support proper lexical scoping. It's object model is broken. It doesn't use proper abstractions in the standard library. Etc. These can all be fixed by compiling another language to Javascript, but then you lose on the performance side.