Albert Barnes on Romans 3:15

Their feet, etc. The quotation in this and the two following verses is abridged or condensed from Isa 59:7,8. The expressions occur in the midst of a description of the character of the nation in the time of the prophet. The apostle has selected a few expressions out of many, rather making a reference to the entire passage, than a formal quotation. The expression, "their feet are swift," etc., denotes the eagerness of the nation to commit crime, particularly deeds of injustice and cruelty. They thirsted for the blood of innocence, and hasted to shed it, to gratify their malice, or to satisfy their vengeance.

(v) "feet are swift" Ps 10:7