Psalms 57:2 - Matthew Poole's English Annotations on the Holy Bible

Heb. that performeth (or perfecteth, or finisheth, as this word is rendered, Psalms 138:8, i.e. will certainly perform or finish) for, or towards, or concerning me. He doth not express what he performeth, or perfecteth, or fulfilleth, but leaveth it to be understood, as being easy to be understood. He performeth, or perfecteth, to wit, all that he hath promised; engageth himself to perform what he hath begun to do, or what is yet to be performed; it being usual in the Hebrew language to understand a verbal noun after the verb. He implies that God is not like men, who make large promises, but either through inability, or carelessness, or unfaithfulness, do not perform them, but will certainly be as good as his word.