The save is made with an Intelligence of 1 i.e. -5 + proficiency bonus
If it's the second, wouldn't this make Feeblemind essentially impossible to save from without a nat 20?
The person who casts Feeblemind is at least 15th level (+5 or +6 proficiency) and probably has a +5 stat bonus giving a DC of 18 or 19. It is possible that the person has stats higher than 20 through magic items - the maximum being 30 of course which would give a DC of 23 or 24.
A natural 20 of a saving throw is not an automatic success - that only applies to Attack rolls. Therefore you need to be at least 5th (DC18) or 9th (DC19) level to ever recover. If you are lower level than this you will never recover - if you can save on a 20 or less you will recover eventually.
This seems like it could wreak a lot of havoc on a small village without anyone capable of casting Greater Restoration?