Adding comments to describe applied math to a "fundamentals" "7 kyu" kata may increase votes on it.
After checking it out: exponents from prime factors?

This warning just indicates you stated as mutable a variable your code cannot mute.

You are comparing the speed of interpreted code without any optimisation. If you compare compiled code with optimisations turned on, then yours is still faster. If however, you modify the bound in my solution to be sqrt x rather than x then it is as fast as yours.

The reason it is upvoted as best practices is because it is best practice to write declarative Haskell programs.