This article answers your questions.
TL;DR?
Threading in Python is inefficient because of the GIL (Global Interpreter Lock) which means that multiple threads cannot be run in parallel as you would expect on a multi-processor system. Plus you have to rely on the interpreter to switch between threads, this adds to the inefficiency.
asyc/asyncio allows concurrency within a single thread. This gives you, as the developer, much more fine grained control of the task switching and can give much better performance for concurrent I/O bound tasks than Python threading.
The 3rd approach that you don’t mention is multiprocessing. This approach uses processes for concurrency and allows programs to make full use of hardware with multiple cores.