The Fibonacci sequence can be computed using matrix exponentiation: $\begin{pmatrix} F_{n+1} \\ F_n \end{pmatrix} = \begin{pmatrix} 1 & 1 \\ 1 & 0 \end{pmatrix}^n \begin{pmatrix} F_1 \\ F_0 \end{pmatrix}$ with $F_0 = 0, F_1 = 1$. Using fast exponentiation to compute $F_n$, what is the time complexity?