Why did they say "Yachishah Ma'asehu"?
Rashi: Let Him quickly bring the threatened punishment!
Radak: When the Navi tells them how they will be punished for their evil, they do not believe it. They say, our Nevi'im say that it will not come!
Malbim: The Navi predicted future punishments. They said, those are for the distant future. Hashem should bring the punishment quickly, and then it will prove that it is via Hashgachah!
Why did they say both "Yemaher" and "Yachishah"?
Malbim: He should bring it soon, and it should be sudden. If it comes slowly, we can attribute it to chance!
What will we see?
Rashi: Whose word will be fulfilled.
What is "Atzas Kedosh Yisrael"?
Radak: What He counseled to bring upon us is because He is Kedosh Yisrael. How will He do evil to us? (They thought that their deeds are good - "Hoy ha'Omerim la'Ra Tov" (20).)
Malbim: Regarding Hashem, "Etzah" always refers to the purpose for His acts. E.g. if a country rebelled against the king, and he takes counsel how to suppress it, send soldiers and make them submissive, sending the troops and the war are the acts to fulfill the counsel. When he suppresses the revolt, his counsel was fulfilled. Hashem does not need counsel and strategies. 'Etzah' is only His desired purpose. When He decrees punishment on a nation, His purpose is so they will recognize His Elokus and ability. Since His purpose is Teshuvah, He must bring it quickly, so we will understand that He does everything.