Recursion is difficult
I once advised a Professor that rather than staying with his very old Sparc box running Solaris under x86 was a better idea.
He bought a new machine and was not very happy as he said it was far slower than his old machine.
I said I couldn't see why but he was the respected professor of computer science.
A couple of days later he admitted there was an infinite loop in his Prolog code and once this was sorted it really was very fast.