But do not overlook this one fact, beloved, that with the Lord one day is as a thousand years, and a thousand years as one day. 9 The Lord is not slow to fulfill his promise as some count slowness, but is patient toward you, not wishing that any should perish, but that all should reach repentance. 2 Peter 3:8-9 (ESV)
(For a better and more recent defense see here.)
This is one of what is called the “Arminian Big Three.” And it is huge when you don’t consider the context. All you see is that God doesn’t want anyone to go to hell, yet somehow being the Almighty, He is not able/willing to save them, but instead sends so many to hell, because they sin against Him. What people mostly miss is that this passage (v 9) actually is not referring to non-believers or the entire human race, but to God’s elect and we will see why. (Please understand that I am not saying that God loves sending people to hell, no, I totally believe Ezek 18:23, 32. But what I believe is that God is glorified in the damnation of the reprobate indeed, Prov 16:4; Rom 9:22).
The first thing we need to examine is to whom all these words refer to (you, any, all). It is clear from the greeting of Peter’s second letter to whom this letter is directed, “…To those who have obtained a faith of equal standing with ours by the righteousness of our God and Savior Jesus Christ” (1:1), thus believers, God’s elect. 2 Pet 3:1 says that those recipients have had received another letter from Peter, that is 1 Peter and we directly see in v 1, “…To those who are elect exiles…” We see again at the beginning of 2 Pet 3:8 that Peter is talking to the “beloved,” a term used referring to Jesus or the believers. Thus we have established that the context and the audience to whom these passages are directed are fellow believers of Peter. In chapter 3 Peter warns his fellow believers about those who will come scoffing about the Second Coming, that it has not yet happened yet Jesus said that He will come soon. He tells them that this present Universe is stored up for wrath (v 7); time is nothing with God (v 8); God is patient toward His sheep, waiting for the ones who yet have to be born and/or be saved, so the Lord is patient toward His own and He’s not willing that any of them perish, but all of them come to Him (v 9).
In 2 Peter 3, the Christians – all God's elect are represented by Peter's audience as His beloved, even when they were dead in trespasses God loved them (Eph 2:1-10) and in love predestined them (Eph 1:3-6). It is for their sake that God is delaying the Parousia of our blessed Savior. God is waiting until the number of His elect is complete then He will send the Savior to judge the world in righteousness.
John MacArthur says the following in the ESV MacArthur Study Bible: 
2 Pet. 3:9 not slow. That is, not loitering or late (cf. Gal. 4:4; Titus 2:13; Heb. 6:18; 10:23, 37; Rev. 19:11). patient toward you. “You” is the saved, the people of God. He waits for them to be saved. God has an immense capacity for patience before he breaks forth in judgment (cf. 2 Pet. 3:15; Joel 2:13; Luke 15:20; Rom. 9:22; 1 Pet. 3:15). God endures endless blasphemies against his name, along with rebellion, murders, and the ongoing breaking of his law, waiting patiently while he is calling and redeeming his own. It is not impotence or slackness that delays final judgment; it is patience. not wishing that any should perish. The “any” must refer to those whom the Lord has chosen and will call to complete the redeemed, i.e., the “you.” Since the whole passage is about God’s destroying the wicked, his patience is not so he can save all of them, but so that he can receive all his own. He can’t be waiting for everyone to be saved, since the emphasis is that he will destroy the world and the ungodly. Those who do perish and go to hell, go because they are depraved and worthy only of hell and have rejected the only remedy, Jesus Christ, not because they were created for hell and predetermined to go there. The path to damnation is the path of a non-repentant heart; it is the path of one who rejects the person and provision of Jesus Christ and holds on to sin (cf. Isa. 55:1; Jer. 13:17; Ezek. 18:32; Matt. 11:28; 23:37; Luke 13:3; John 3:16; 8:21, 24; 1 Tim. 2:3–4; Rev. 22:17). all should reach repentance. “All” (cf. “you,” “any”) must refer to all who are God’s people who will come to Christ to make up the full number of the people of God. The reason for the delay in Christ’s coming and the attendant judgments is not because he is slow to keep his promise, or because he wants to judge more of the wicked, or because he is impotent in the face of wickedness. He delays his coming because he is patient and desires the time for his people to repent.
The ESV Reformation Study Bible explains: 
3:9 as some count slowness. See v. 4.
patient . . . all should reach repentance. Peter’s Christian readers must realize that the apparent delay of divine judgment is a sign of God’s forbearance and mercy toward them, particularly toward the believers in their midst who have been confused and misled by the false teachers. The repentance in view, for the sake of which God delays judgment, is that of God’s people rather than the world at large. God is not willing that any of His elect should perish (John 6:39).
The HCSB Study Bible explains: 
3:9 The Lord has not yet returned, says Peter, because He is patient with you, not wanting any to perish. "You" is variously interpreted as a reference to the letter's Christian recipients (identified in 1:1) or else more broadly as all people. In chapter 1 "you" and "your" both refer back to the recipients identified in 1:1 (see 1:2,4,5,8,10,11,12,13,15,16,19,20). Peter's later use of "dear friends," (3:1,8,14,17) seems also to point back to those identified in 1:1.
What Matthew Henry said about 2 Peter 3:9: 
That what men count slackness is truly long-suffering, and that to us-ward; it is giving more time to his own people, whom he has chosen before the foundation of the world, many of whom are not as yet converted; and those who are in a state of grace and favour with God are to advance in knowledge and holiness, and in the exercise of faith and patience, to abound in good works, doing and suffering what they are called to, that they may bring glory to God, and improve in a meetness for heaven; for God is not willing that any of these should perish, but that all of them should come to repentance. Here observe, 1. Repentance is absolutely necessary in order to salvation. Except we repent, we shall perish, Luke 13:3; Luke 13:5. 2. God has no delight in the death of sinners: as the punishment of sinners is a torment to his creatures, a merciful God does not take pleasure in it; and though the principal design of God in his long-suffering is the blessedness of those whom he has chosen to salvation, through sanctification of the spirit, and belief of the truth, yet his goodness and forbearance do in their own nature invite and call to repentance all those to whom they are exercised; and, if men continue impenitent when God gives them space to repent, he will deal more severely with them, though the great reason why he did not hasten his coming was because he had not accomplished the number of his elect. "Abuse not therefore the patience and long-suffering of God, by abandoning yourselves to a course of ungodliness; presume not to go on boldly in the way of sinners, nor to sit down securely in an unconverted impenitent state, as he who said (Matt. xxiv. 48), My Lord delayeth his coming, lest he come and surprise you;"
Here is what John Gill said: 
but is longsuffering to us-ward: not to all the individuals of human nature, for the persons intended by us are manifestly distinguished from "some men" in the text, and from scoffers, mocking at the promise of Christ's coming, in the context, 2Pe 3:3; and are expressly called beloved, 2Pe 3:1; and God's longsuffering towards them is their salvation, 2Pe 3:15, nor is it true of all men, that God is not willing that any of them should perish, and that everyone of them should come to repentance, since many of them do perish in their sins, and do not come to repentance, which would not be the case, if his determining will was otherwise; besides, a society or company of men are designed, to which the apostle himself belonged, and of which he was a part; and who are described, in his epistles, as the elect of God, called out of darkness, into marvellous light, and having obtained like precious faith with the apostles; and must be understood either of God's elect among the Jews, for Peter was a Jew, and they were Jews he wrote to; and then the sense is, that the delay of Christ's coming is not owing to any slackness in him, but to his longsuffering to his elect among the Jews, being unwilling that any of that number among them should perish, but that all ...