My previous comment was assuming 32-bit Python on Windows; now I see it is 64-bit.

At the end of the video, I see that the cursor stops blinking, and you can’t do anything (closing the window seems to do nothing). So maybe the OS has crashed or frozen, or maybe it is just being really slow.

In the video I also see you have about 8 GB of RAM. The Python strop code should try to allocate a temporary buffer of 0xEAAA * 2 * 0xAAAA bytes = 5.2 GB, and then a second 5.2 GB object which is the final string result. It copies the temporary buffer into the string object, so it will temporarily require 10.4 GB allocated. Does Windows use swap memory? I suspect what you see is normal Windows behaviour when it runs low on memory, and it is not Python’s fault.