IDL operations are not particularly slow. Last I measured, a call from JS into C++ is between 10 and 40 CPU instructions of fixed overhead depending on the exa

IDL operations are not particularly slow. Last I measured, a call from JS into C++ is between 10 and 40 CPU instructions of fixed overhead depending on the exact thing being called (method vs getter vs setter; they have slightly different costs) and the browser involved. At least once you get into the top JIT tier; baseline jit and interpreter can involve more overhead, obviously. There's additional cost for dealing with the call arguments, too; this varies widely by type of argument (e.g. an options object is clearly more expensive to deal with than a boolean, or even than a list of arguments containing the same information as the options object) and somewhat less widely by browsers.