@Yalu12 I disagree, the empress is not only fighting against those nobles but also against those who helped them.
The corrupt nobles have many "ties" and she did well to investigate every little thing, a death sentence could not be given without all the evidence also for fairness to those who have been harassed by these nobles.
20 chapters are right, at least in this case.
By pretending, the empress was able to let the corrupt nobles lower their guard, if she had hastily given the sentence her image would have suffered.