The cost of transpiling es2015 isn't very visible here, because the "vanilla es6 TodoMVC" example used [1] doesn't rely heavily on ES6 only features, such as generator functions etc., that aren't just sugar on top of ES5. Such features are expected to decrease the performance of the generated code significantly and should be taken into account when a transpilation step is involved.

[1] https://github.com/tastejs/todomvc/tree/master/examples/vani...