At the moment it is not possible for a teacher to delay posting of peer-reviews. In some cases this could have pedagogical advantage. It would be helpful, for instance, when teachers would want to post peer-reviews along with their own comments, or when they would like to hold a class where they get a chance to look at common mistakes with students before releasing peer-reviews and teacher's own comments. This would even be beneficial for students in such circumstances.
The solution could be either a dropdown menu/radio buttons in the assignment settings where a teacher could select time to release the peer-reviews (with default "immediately", and the select time option appearing if "delayed" is chosen), or some kind of button that would post all of the peer-reviews that would accessible from the assignment page or Speedgrader. This would make at least two pedagogical scenarios described above possible.
