Dragging messages from the shapes panel is really a slow way of drawing diagrams.
It would be nice to have a keyboard shortcut for dropping a message exactly to the position of cursor. (Thanks to message autosnapping, it usually gets connected to required instances.)
Define two keyboard shortcuts, one for each direction (left-to-right, right-to-left). The user should be able to invoke the shortcut just by their left hand (because they hold mouse in right hand, mostly, which specifies the location to drop the message).