Does bitset optimize time ? or memory only?

Let there is an array ara[n] and a bitset a , both has size n. . if we access the array with a for loop as for(int i=0;i<n;i++)ara[i] the complexity is O(n). Now if we access the bitset as for(int i=0;i<n;i++)a[i] then what is the complexity? O(n) or O(n/32)??