The context of this phrase of the apostle Peter is clear: the matter is the Second Coming of Christ, which the first Christians expected literally from day to day. Jesus promised to return, and for Christians it was obvious that His return is happiness, the thing they desired with all their heart. At times they could not understand why this promise was not being fulfilled, why the Lord was delaying. Had He changed His mind? Did He not want to come? Peter explains that this is not delay, and that the Lord has not changed His mind. His goal is not to come, but to save. Therefore He is patient, taking into account the pace of the changes taking place in people. It is not He who delays, but we who delay: we delay with our own conversion and with our help to other people in their coming to repentance.
But this verse can also be discussed in a broader context. We always have requests for God; we pray about our problems and at times do not see an answer to our prayers. We are perplexed: why is God silent, why does He delay in fulfilling our requests? This again is not God's delay, but His long-suffering. He patiently waits until we ask not for what we want for ourselves, but for what He wants for us. He wants the answer to our prayer to be truly good, good for us and for everyone He loves. Someone once said that we should remember that when we pray for a favorable wind, it will be a headwind for someone else. God does not delay in answering our prayers; we delay in answering His call to love one another.