“It turns out that, with a clever trick, we can remove this extra frame, simplify the V8 codebase and get rid of almost the entire overhead.”
JavaScript lets you call any function with the wrong number of arguments. Frameworks lean on that for optional parameters, and V8 paid for it with an extra stack frame on every mismatched call. Deleting the frame made those calls nearly free. Sloppy language design costs a decade of compiler engineers cleaning up after it.