Why can using unsigned short be slower than using int?

Question

On this webpage, the comments on unsigned short state:

Used to reduce memory usage (although the resulting executable may be larger and probably slower as compared to using int

Why is this?

Answer 1

I think the wording "probably slower" is too hard.

A theoretical fact is:

Calculations are done with at least int size, e.g.

short a = 5;
short b = 10;
short c = a + b;

This code snippet contains 3 implicit conversions. a and b are converted to int and added. The result is converted back from int to short. You can't avoid it. Arithmetic in C++ uses at least int size. It's called integer promotion.

In practice most conversions will be avoided by the compiler. And the optimizer will also remove many conversions.