What causes folders to load slowly when folder type is "Pictures"?

I know that when folder is optimized to be 'Pictures' (in the Properties>Customize tab) it loads slowly. For example the Download folder would low very slowly if it's not set to be "General items"

But why exactly does that happen? Or that's more of a code thing/bug and only Microsoft software people can answer that?

A lot depends if you have thumb nails set if you do it must load all the files to get image if it's set to details it only needs file names etc

If information such as image dimensions is displayed this would you require reading the file header for each file. This will take considerable time, particularly with large folders.