Multithreading does not change speed

Question

I am currently working on speeding up one of my c++ program, and trying to discover the beautiful world of multithreading.

std::vector<std::thread> threadPool;
threadPool.clear();
for(int t = 0; t < nbThreads; ++t){
    threadPool.push_back(std::thread(myFunction, std::ref(rep), std::ref(branch1));
}
for(auto t = threadPool.begin(); t < threadPool.end() ; ++t){
    t->join();
}

What I don't understand is that whatever the number of threads I use, the speed does not change! (about 30sec).

To sum up : If my code is wrong, what is wrong? If it is not wrong, in what case should I use multithreading to speed up the computation?

Edit : I run on a i7-4810MQ machine

Edit 2 : here is what myFunction does (it parses xml files)

void myfunction(DIR*& rep, struct dirent*& branch1){
mtx.lock();
while ((branch1 = readdir(rep)) != NULL){
    mtx.unlock()

    TiXmlDocument doc(branch1->d_name);
    if(doc.LoadFile()){
            //parse file
    }

    mtx.lock();
}
mtx.unlock();
}

Answer 1

There's a good chance that your code is limited by file I/O. There's also a good chance that your XML parser makes lots of memory allocations, and memory allocations may slow down if many threads allocate memory simultaneously.