The problem is that "with" statements basically disable lots of JIT optimizations in actual implementations, with browsers either falling back to a much slower

The problem is that "with" statements basically disable lots of JIT optimizations in actual implementations, with browsers either falling back to a much slower baseline jit or running the function containing "with" entirely in an interpreter. So they _can_ be used effectively if one really knows what one is doing, but they're a bit of a footgun.