Python's a bit faster than Bash.

For what? Is it faster than (g)awk? How about sed? How about dd? How about any of the thousands of tools in userspace that you wrap in bash and are faster than python at the same task? Use the right tool is being lost in the 'you are too stupid to make a good decision, do it this way' generation.

It depends, of course! But usually the issue with shell scripts is that they create many new processes, which has high overhead. In the instances you're spending more time in dd than bash, in Python you're probably inside an in-process memcpy.