I did not perform bench marking, But can get idea from following
Although implemented in C++, Wt’s main focus or novelty is not its performance, but its focus on developing maintainable applications and its extensive library of built-in widgets. But because it is popular and widely used in embedded systems, you will find that performance and foot-print has been optimized too, by virtue of a no-nonsense API, thoughtful architecture, and C++ …
given in webtoolkit tutorial