LeetCode 11 Container With Most Water - Why Naive Fails

Check every pair of lines $(i, j)$ and compute area as min(height[i], height[j]) × (j - i). Track the maximum.

With $n = 10^5$, you have roughly $5 \times 10^9$ pairs. That's far too slow.

You need a way to skip pairs that can't be optimal. What property tells you a pair is definitely worse than another?