You can speed-up caption writing by copying selected content from any article element to caption.

In version 1.5

Select some text in any article element, and then on the Type menu, point to Fill with, and then select Photo Caption with Highlighted Text.

In version 1.4 and older

Select some text, and then call the ArticleCopyTxtTo("photoCaption") system script.

You can assign such script to a shortcut key, menu command or custom icon.