You can create a dictionary to store the index and value of elements present in the big list. Then, you can search the dictionary to find the indices of the desired elements. Searching a dictionary is extremely fast. Check the following example:
>>> sl
[8, 6, 4, 5, 0, 2, 7, 1, 3, 9]
>>> bg
[33, 15, 28, 41, 27, 30, 46, 3, 25, 17, 2, 36, 29, 20, 22, 8, 47, 39, 7, 19, 11, 5, 12, 42, 24, 16, 9, 43, 13, 45, 21, 37, 18, 0, 31, 44, 23, 1, 32, 40, 14, 34, 48, 6, 4, 35, 38, 26, 49, 10]
>>> tmp_dict = {}
>>> for i,v in enumerate(bg):
... tmp_dict[v] = i
...
>>> idx=[]
>>> for e in sl:
... idx.append(tmp_dict[e])
...
>>> idx
[15, 43, 44, 21, 33, 10, 18, 37, 7, 26]