The Lord is not slow in keeping His promise as some understand slowness, but is patient with you, not wanting anyone to perish but everyone to come to repentance.
The Lord's apparent slowness is really patience meant to lead people to repentance.
This verse interprets delay as an expression of mercy.
8Beloved, do not let this one thing escape your notice: With the Lord a day is like a thousand years, and a thousand years are like a day. 9The Lord is not slow in keeping His promise as some understand slowness, but is patient with you, not wanting anyone to perish but everyone to come to repentance. 10But the Day of the Lord will come like a thief. The heavens will disappear with a roar, the elements will be destroyed by fire, and the earth and its works will be laid bare. 11Since everything will be destroyed in this way, what kind of people ought you to be? You ought to conduct yourselves in holiness and godliness 12as you anticipate and hasten the coming of the day of God, when the heavens will be destroyed by fire and the elements will melt in the heat. 13But in keeping with God’s promise, we are looking forward to a new heaven and a new earth, where righteousness dwells.