It can be slow for basically the same reason as in bug 1331136: We won't flatten the rope because of the optimization in JSString::getChar(), which means we always call into the runtime to retrieve char. Example benchmark: var q = 0; var dt = 0; for (var i = 0; i < 1000000; ++i) { var s = "a".repeat(15); s = s.split("").join(""); // flatten s = s + s; // rope var t = dateNow(); for (var k of s) ++q; dt += dateNow() - t; } print(dt, q); This µ-benchmark currently completes in ~525ms for me, adding support to load a char from right-hand side of a rope to MacroAssembler::loadStringChar() (see attached patch) would improve it to ~380ms. The question is now whether or not it's worth it to handle the rhs of a rope in MacroAssembler::loadStringChar()?