Rashi

לי נקם ושלם means, with Me is prepared and held in readiness the punishment of vengeance, and it will recompense them according to their doings. As regards the word ושלם, it means that vengeance will pay them their recompense (וְשִׁלַּם is therefore a verb with the Vau conversive, the subject being the noun נקם which precedes it). — But some explain the word וְשִׁלַּם as a noun, having the same meaning as שִׁלּוּם “recompense”, when it would be of the same grammatical form as the noun in the phrase (Jeremiah 5:13) “And the דִּבֵּר is not in them”, where it has the meaning of דִּבּוּר, “Divine utterance”. And when will I recompense them?
לעת תמוט רגלם AT THE TIME WHEN THEIR FOOT SHALL TOTTER — i.e. when the merit of their ancestors upon which they are relying (which is, as it were, their foot, their support) will become exhausted.
כי קרוב יום אידם FOR THE DAY OF THEIR CALAMITY IS AT HAND — i.e. as soon as I shall desire to bring about the day of their calamity, it will be near at hand and held in readiness by Me — that I may bring it by any of the many agencies I have.
וחש עתדת למו means, AND THE THINGS THAT ARE TO COME (עתידות) UPON THEM WILL COME QUICKLY (חש).
וחש — The word חש has the same meaning as the verb in (Isaiah V. 19) “Let him make speed and hasten (יחישה)”. — So far Moses exhorted them with words of reproof, so that this song should be witness against them, i.e., that when the punishment would come upon them they would know (acknowledge) that I told them about this from the very beginning. From here onwards he exhorted them with a statement of the comfort that would come upon them when their punishment would have ended; this being similar to all that is stated above (30:1—2) “And it shall come to pass when [all these things] are come upon thee, the blessing and the curse, … then the Lord thy God will turn they captivity, etc.”