Worst Case: Chain Tree

Consider a chain: $1 \to 2 \to 3 \to. \to n$. Each node has one child except the leaf. Node $n$ has set size $1$. Node $n-1$ merges with $n$, giving size $2$. Node $n-2$ merges with $n-1$, giving size $3$. You keep copying larger and larger sets.

Total operations: $1 + 2 + 3 +. + n = O(n^2)$. For $n = 100,000$, this is $5$ billion operations. Your solution times out. The pattern happens whenever you always merge into the smaller set.