Using the task scheduler is usually the best approach to threading for performance, however there are cases when the task scheduler is not appropriate. The task scheduler is intended for high-performance algorithms composed from non-blocking tasks. It still works if the tasks rarely block. However, if threads block frequently, there is a performance loss when using the task scheduler because while the thread is blocked, it is not working on any tasks. Blocking typically occurs while waiting for I/O or mutexes for long periods. If threads hold mutexes for long periods, your code is not likely to perform well anyway, no matter how many threads it has. If you have blocking tasks, it is best to use full-blown threads for those. The task scheduler is designed so that you can safely mix your own threads with Intel® Threading Building Blocks tasks.
For more complete information about compiler optimizations, see our Optimization Notice.