Kiss the Son, lest he be angry and you be destroyed in your way, for his wrath can flare up in a moment.
Psalm 103:8 (also Exodus 34:6-7)
The LORD is compassionate and gracious, slow to anger, abounding in love.
God's response to sin is phased. When someone sins, God of course condemns it. Yet he is also patient with us, "not wanting anyone to perish, but everyone to come to repentance" (2 Pet 3:9) - he gives us time to acknowledge our sin and repent of it before he responds with punishment. However, when time has passed and he decides to punish, he acts quickly. For instance, he waited to punish Sodom and Gomorrah until they had sinned greatly (Gen 18:20-21), but once their grace period ran out, he acted swiftly (Gen 19:15-22). Thus, God is slow to anger in that he gives people a long time to repent, yet he is quick to condemn sin and quick to punish when the time comes.
One of the meanings of wrath given by the Merriam-Webster dictionary is "retributory punishment for an offense or a crime: divine chastisement," so Psalm 2:12 is probably referring to the speed of punishment.
Other responses (offsite)