As a follow-up question, reading this recipe the question comes to mind is that what happens if I add salt to the oil instead of sprinkling salt on chips after they're removed from the oil? Sprinkling salt on chips would make them too salty or not-as-salty-as-desired, so I prefer to add salt to the oil. But I've heard it might make the cooking time longer, is it true?
Potato chips have to be fried in lots of oil. If you are using little, you are doing it wrong and your chips are less tasty than they could be.
As Preston Fitzgerald mentioned in a comment, salt doesn't dissolve in oil. It could be that it will fall to the bottom. Alternatively, the convection in the hot oil could move it around. But still, the salt will tend to stay in the oil and not cling to the potatoes. Even if an oiled flake meets the potato, it will not stick, the oil will lubricate it enough to prevent sticking. If you salt the raw potatoes instead, the salt will dissolve in their wetness and coat them evenly.
Your popcorn is entirely different. There, your small amounts of oil, including any salt floating in them, will cling to the popcorn. This is different from frying, where only a tiny fraction of the oil will stay on the potato. Besides, you can't salt corn kernels before popping - the salt will just fall off the hard dry surface. The potato is perfectly salted before.
Another way to achieve what you're looking for...
My mother in law will cut her beans and add salt to them. Because the beans have been washed in water, there's enough water to dissolve the salt. She'll then deep fry the beans for a specific dish, and when the beans are done frying, they're salty enough. She does this with eggplant and bitter gourd as well, as have I. The bitter gourd dish I make with the deep fried salted bitter gourd needs no further salt added since there's enough in the bitter gourd. And, as an aside, those bitter gourd chips are mighty tasty. :)
I don't know how you do your chips, but it seems to me you could do something similar to your chips. Add salt, mix them around a bit, then deep fry them. You don't want the chips to be soggy, but wet enough that the salt will dissolve.
The texture of the salt used is key here. Try using dust-fine salt (use a mortar or grinder) instead of the usual, still relatively coarse table or kosher salts: it will actually adhere to the fried food instead of being accidentally eaten alongside, and you have more control over the dosage.
And no, salt in oil might help dehydrate the food a little quicker, but could also create new problems (spoiling the oil, creating water pockets and splattering...) - but it won't be effectively dissolved in it or distributed to the food.
The professional catering services will leave the freshly cut potato chips to soak in a sugary/saline solution for 12 hours prior to frying. If you want to add even more finesse, McDonald's is rumored to add additional additives such as phenethyl alcohol & dimethicone to its French Fries.