There is no I-485 filing when parents are outside the US, you just file I-130 now and they get interviewed and approved at a US consulate in their country. It is somewhat faster if you file in the US but you cannot file immediately after they arrive here.
If they are in their country, it takes 6-8 months from the time you file I-130 to the time they are invited for interview. If you apply AFTER they arrive here, it takes 5-7 months from end to end, plus the initial waiting period of around 3-4 months.
In summary, if you want them to get a green card as quickly as possible, file I-130 now and opt for consular processing in their home country. However they cannot travel to US after I-130 is filed.
