In general don't use threads, especially if you've no idea how to do proper synchronization etc, etc.
Parallel programming is one of the harder topics and especially since C++ itself only comes with a limited amount of out-of-the-box tools.
Unless you're on a very large project, the time you'll need to invest in learning proper multithreading and maintaining that code base is way larger than the gain you'll get from it. Multiple threads doesn't automatically mean double the processing time, it's often even the other way around, due to bad synchronization or other challenges.