The quick and dirty way to check your usage is to replace “who” with “he” and “whom” with “him.” If it makes a functional sentence, you’re probably right.

“Whom is quicker?” becomes “Him is quicker.” Not good.

“Who is quicker?” becomes “He is quicker.” That works.

By contrast: “This graffiti was done by whom?” becomes “This graffiti was done by him.”