2 Peter 3 mentions scoffers and the wicked. God's promise is not slack. If you notice, the context is the destruction of the wicked, scoffers, Noah's time is mentioned. So the promise here to the beloved elect is God is not slack about His promise to destroy the wicked and to let the righteous inherit the earth. This is not about the salvation of the wicked, it is a promise of their destruction.
Where it mentions God's long suffering patience, this is referring to His elect, God waits patiently for those who He knows will believe in Christ. For their sake (the sake of the wheat) He tolerates the tares for a season of time. Key to understanding who God is longsuffering toward is in the 'beloved' and 'us', that is His beloved elect whom he knows will repent. He is not willing that any of them perish. This is not about the wicked unbelieving world that he is longsuffering towards. I know that flies smack dab in the face of what many are taught, but I know you will see what I also see too.
Matthew 13
24 Another parable He put forth to them, saying: “The kingdom of heaven is like a man who sowed good seed in his field; 25 but while men slept, his enemy came and sowed tares among the wheat and went his way. 26 But when the grain had sprouted and produced a crop, then the tares also appeared. 27 So the servants of the owner came and said to him, ‘Sir, did you not sow good seed in your field? How then does it have tares?’ 28 He said to them, ‘An enemy has done this.’ The servants said to him, ‘Do you want us then to go and gather them up?’ 29 But he said, ‘No, lest while you gather up the tares you also uproot the wheat with them. 30 Let both grow together until the harvest, and at the time of harvest I will say to the reapers, “First gather together the tares and bind them in bundles to burn them, but gather the wheat into my barn.” ’ ”
1 Beloved, I now write to you this second epistle (in both of which I stir up your pure minds by way of reminder), 2 that you may be mindful of the words which were spoken before by the holy prophets, and of the commandment of [a]us, the apostles of the Lord and Savior, 3 knowing this first: that scoffers will come in the last days, walking according to their own lusts, 4 and saying, “Where is the promise of His coming? For since the fathers fell asleep, all things continue as they were from the beginning of creation.” 5 For this they willfully forget: that by the word of God the heavens were of old, and the earth standing out of water and in the water, 6 by which the world that then existed perished, being flooded with water. 7 But the heavens and the earth which are now preserved by the same word, are reserved for fire until the day of judgment and perdition of ungodly men.
8 But, beloved, do not forget this one thing, that with the Lord one day is as a thousand years, and a thousand years as one day. 9 The Lord is not slack concerning His promise, as some count slackness, but is longsuffering toward [c]us, not willing that any should perish but that all should come to repentance.
