How to decrease the delay to drag and drop files into windows via Dock?
In the Finder's preferences, there is an option at the bottom of the General pane for “Spring-loaded folders and windows” that seems to control this. You can adjust the delay using the slider there, and also (as it notes) press the spacebar when dragging an item to invoke Exposé immediately.

It's not a hidden preference (in the sense that you need defaults to get at it) so much as a preference in a somewhat unexpected place. ;)