I'd like to know what the subset of JavaScript is that causes the least performance problems. Like, I wish all programming languages came with a big wall chart of onion layers, showing "oh this core kernel nugget is the fastest (and probably least expressive) subset; and then this next outer layer adds a feature that when used incurs a penalty; and then the next layer out from that; and then 'eval' just dooms us utterly." Then we could as humans better understand our own code, and we could also try to train our compilers to know.