Obviously. But the seconds part is the same, unless they have customized its typeface or background.

So, it only needs to be rendered once, and then blitted in. Or, you can pre-render ten digits, and blit them up as needed.

Caching of the rasterization is already done at the font level. Attempting to cache the rendered seconds (even ignoring desktop preferences as you say, which is already a non-starter) seems like an engineering problem that massively outweighs saving the few dozen bytes for that portion of the screen