I have been working on python programming language, python is arguably a slow language due to many factors, out of which include the lack of multithreading features, if it doesn't support multithreading, then why does it have a threading module?
Python's single-threaded nature is due to the GIL (Global interpreter lock). When people refer to python being single threaded, they are describing how python operates when not using the threading or multiprocessing libraries. You can still make python use more threads, or spin up multiple processes, but for each instance of the code that you are running, it will only be using a single thread.
Javascript for example can make use of multiple threads and doesn't require any additional "work" to make this happen.
Check out this video for some more info: https://www.youtube.com/watch?v=m2yeB94CxVQ