Less time to mark letter as read

Some email clients able to mark letter as read after more then zero but less then one sec. 1 sec is too long for me to get the subject of most emails, but 0 sec makes it easy to accidentally mark letter as read while it is unread by myself.
So my request is to let user set time to mark letter as read between 0 and 1 sec.
I use 0.5 sec and it looks like near-to-perfect time for me.