Deuteronomy 10:10 and 2 Peter 3:9 lend lucidity to each other.

Deuteronomy 10:10

“I myself stayed on the mountain, as at the first time, forty days and forty nights, and the Lord listened to me that time also. The Lord was unwilling to destroy you."

2 Peter 3:9

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.

The Israelites wandered around in their wilderness for forty years. They repeatedly sinned and turned away from the Lord

What could have happened was:

Israelites sin--be punished--die.

What the Lord wanted instead was:

Israelites sin--be disciplined/learn lessons--gain promise.

And the same applies for us. If we're wandering around in some kind of wilderness it's not because the Lord is slow in keeping His promises. But because He is unwilling to see us perish. The Lord making us wait is actually Him giving us a chance to change, grow in Him and be blessed.