Carl Friedrich Keil and Franz Delitzsch Old Testament Commentary

In consequence of this resolution, the Lord let these tribes (those mentioned in Jdg 3:3) remain at rest, i.e., quietly, in the land, without exterminating them rapidly. The expression מהר, hastily, quickly, i.e., according to the distinct words of the following clause, through and under Joshua, appears strange after what has gone before. For what is threatened in Jdg 2:21 is not the suspension of rapid extermination, but of any further extermination. This threat, therefore, is so far limited by the word "hastily," as to signify that the Lord would not exterminate any more of these nations so long as Israel persisted in its idolatry. But as soon as and whenever Israel returned to the Lord its God in true repentance, to keep His covenant, the Lord would recall His threat, and let the promised extermination of the Canaanites go forward again. Had Israel not forsaken the Lord its God so soon after Joshua's death, the Lord would have exterminated the Canaanites who were left in the land much sooner than He did, or have carried out their gradual extermination in a much shorter time than was actually the case, in consequence of the continual idolatry of the people.