1 Monty: Grandad! But, weren't you killed in the zeppelin explosion in Berlin?!
2 {scene: double long horizontal panel showing a panorama of Monty, Prof. Jones, and Sallah silently looking up at Minnesota Jones standing on the dock}
3 Minnesota Jones: The boy's still a bit slow, I see.

Scott McCloud writes in his seminal book Understanding Comics:

In learning to read comics we all learned to perceive time spatially, for in the world of comics, time and space are one and the same.

He points out that a long panel conveys a feeling of a longer time than a shorter panel. Especially when used for dramatic pauses like this. You probably felt that without thinking about it (at least I hope you did). Next time you notice a long pause in a comic, take a look at what makes you think it's a long pause. It may be this trick, or something else...