“transactional mental burden” refers to the response time expected from the receiving individual.
In an IM ping , the sender is not sure if the other person is there or not. While the receiver is uncertain about the response time expected out of him.
Another, older example would be how the introduction of voice message technology reduced the “transactional mental burden” of calls made to cell phones. It reduced the anxiety of the caller, as in way they had delivered a message. At the same time the receiver was able to receive the message, albeit with a delay.
In both cases the response time expected from the other side was reduced.