There is no need to go up to the number to know its factor, going up to the sqrt is enough:

def D(x):
    return sum(i**2 + (x / i)**2 for i in range(1, math.floor(x ** 0.5) + 1) if not x%i)

That should speed up quite a bit the computation if you are dealing with big numbers

There is no need to go up to the number to know its factor, going up to the sqrt is enough:

def D(x):
    return sum(i**2 + int(((x / i) ** 2 if i * i != x else 0))  for i in range(1, floor(x ** 0.5) + 1) if not x%i)

That should speed up quite a bit the computation if you are dealing with big numbers