I typically only read the sender & subject line before marking an email as read, because the content of most emails is typically not important to me. This proportion of "typically" is upwards of 80%.
It is annoying to typically mark most of my inbox as read, when the "Mark as read" button is so far (in physical screen distance) from the subject lines which I am scanning.
One solution, which may not be the best, is to have a non-default setting to move the "Mark as read" button adjacent to the "Star conversation" button