Premature optimisation IS the root of all evil. This is not premature optimisation. This is poor data type choice. Premature optimisation would be something like selecting char over varchar because someone heard it's faster and didn't do any evaluations to see if it is or if there's a performance problem in the first place.

Premature optimisation is making weird data type or design choices because 'they're faster' without any testing or any consideration as to whether any imagined or real performance problems exist, resulting in a more complex or strange design that's harder to maintain or even slower than it could be.