Write if else loop in simple way to reduce time complexity

I have the below code

for i in range(index, len(df_2)+1):
  if df_2.loc[i, 'Duration'] == 0:
       df_2.loc[i, 'Duration'] = df_2.loc[i, "idle_hrs"] + df_2.loc[i - 1, "Duration"]

How can i write this in simple way to reduce time complexity? is there a war to write it in list comprehension style?