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

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