The Lord is not slow in keeping his promise, as some understand slowness. Instead he is patient with you, not wanting anyone to perish, but everyone to come to repentance (2 Peter 3:9).
I have discussed this passage elsewhere, but perhaps it is appropriate to at least briefly discuss it here as well. There is ancient variation of this verse, where instead of "he is patient with you," it reads "he is patient with us." This variation does not significantly affect the theology of the verse but does gives further support to my thesis here, namely, that Peter's words were addressed believers and not unbelievers.
Peter could have simply written, "God is patient, not wanting anyone to perish..." Then the "anyone" and "everyone" that follow might more likely include every person, believers and unbelievers alike. However, because he adds the words "with you," he qualifies "anyone" so that we understand him to mean "any one of you, and "everyone" to mean "every one of you." The "you" or "us" (in the variation) refers to believers. In contrast, Peter wrote that the unbelieving "scoffers" are destined for destruction (2 Peter 3:3-7).
So the nature of God's patience is that He is holding back judgment so that every believer that He has destined for salvation would come to repentance. It is the same as God did in Noah's day, when he patiently waited for Noah to finish the ark before bringing the floodwaters. God also patiently waited for Lot to flee Sodom before bringing destruction on that city.
God's patience cannot mean that He was hoping for the wicked to repent, for that would mean, as the heretical "Open Theists" claim, that God does not know what people will decide and do beforehand. But God is omniscient, and as David wrote, "All the days ordained for me were written in your book before one of them came to be" (Psalm 139:16). And if God did know who would reject and accept Him, what would His patience mean then, except that He was patiently waiting for those that He ordained for salvation to actually come to the point in their lives where they repent of their sins and profess faith in Christ.
Paul describes something similar in Romans 9:22-24,
What if God, although choosing to show his wrath and make his power known, bore with great patience the objects of his wrath—prepared for destruction? What if he did this to make the riches of his glory known to the objects of his mercy, whom he prepared in advance for glory— even us, whom he also called, not only from the Jews but also from the Gentiles?
However, in this case, God is patient, not so much in waiting for believers to get saved but He patiently withholds judgment on "the objects of his wrath" because He wants to use them to make His power known and so magnify the glory reserved for those "whom he prepared in advance for glory." Paul gives Pharaoh and Egypt as an example, where instead of destroying them immediately, he withheld destruction so that He might display His power to His chosen people through displays of miracles, plagues and great wonders against Pharaoh and Egypt.
So, this verse does not teach that God is patient in the sense that He wants to save every single person. No, this verse teaches that God wants those that He has chosen for salvation to actually come to repentance and be saved. He patiently awaits for this, and even withholds judgment on the "scoffers" for this reason.