How to decrease the delay to drag and drop files into windows via Dock?

The Dock (as of 10.7+) allows you to drag files over an application icon, invoking application exposé which allows you to drag the file over the window of choice and drop it in where you want.

The delays set for these actions are rather long for me. I already checked the Secrets prefpane, no luck.

Is there a hidden preference to speed them up?

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. ;)