Yes, because of the Global Interpreter Lock (GIL) there can only run one thread at a time. Here are some links with some insights about this:
From the last link an interesting quote:
Let me explain what all that means. Threads run inside the same virtual machine, and hence run on the same physical machine. Processes can run on the same physical machine or in another physical machine. If you architect your application around threads, you’ve done nothing to access multiple machines. So, you can scale to as many cores are on the single machine (which will be quite a few over time), but to really reach web scales, you’ll need to solve the multiple machine problem anyway.