I made a test to compare string operations in several languages for choosing a language for the server-side application. The results seemed normal until I finally tried C++,
I just tested the C++ example myself. If I remove the the call to std::sting::find, the program terminates in no time. Thus the allocations during string concatenation is no problem here.
If I add a variable sdt::string abc = "ABCDEFGHIJKLMNOPQRSTUVWXYZ" and replace the occurence of "ABC...XYZ" in the call of std::string::find, the program needs almost the same time to finish as the original example. This again shows that allocation as well as computing the string's length does not add much to the runtime.
Therefore, it seems that the string search algorithm used by libstdc++ is not as fast for your example as the search algorithms of javascript or python. Maybe you want to try C++ again with your own string search algorithm which fits your purpose better.