The Quadratic Combine

This maximum-subarray-sum solution is correct but slow. The helper crossing uses a double loop, so it costs $\Theta(m^2)$ on a slice of length $m$; everything else in best is $\Theta(1)$.

def best(a, lo, hi):
    if hi - lo == 1:
        return a[lo]
    mid = (lo + hi) // 2
    return max(best(a, lo, mid),
               best(a, mid, hi),
               crossing(a, lo, mid, hi))

What is the total running time on $n$ elements?