问openmp循环在开始时非常快，但后来变得很慢(几乎停滞)

EN

// something before this ...
// filename_list is quite long, 100k+ files

vector<string> some_string_list(filename_list.size(), " ");
int file_count = filename_list.size();

int counter = 0;

#pragma omp parallel for reduction(+:counter) schedule(auto)  
for (int i = 0; i < file_count; i++)
{
  string loop_filename = filename_list[i];  
  counter++;

  // The filename is used to call a class member function and get some data, the file is used for reading
  // This is the heavy part of code
  some_string_list[i] = someclassinstance.getSomeString(loop_filename, param1, param2);
  int temp = counter;
  if(temp % 1000 == 0)
  {
    cout << "." << flush;
  }
}

cout << endl;