Gray scale texture resolution vs Size?

i’m wondering what is the difference between a 8k grayscale and a 1k grayscale
basically is it resolution that cost’s more performance or is it texture size
despite the base color if you are using a alpha or displacement you may want more details and it comes with resolution but grayscale texture are way lighter than base color
for example if it’s a 8k RGB color it’ll be like 20 - 50 mgb but if it’s a grayscale it’s less than 3mgb.
now my question is: is it the resolution that is taking performance or it’s texture size
is it better to use low res texture’s depend on mesh size or it doesn’t matter as long it’s a light size texture