Few questions

My apologies in advance if this thread is in the wrong subforum, I haven't been on these forums in months and am not too familiar with these, but this is the closest thing I could find while looking around. If it does happen to be misplaced, would anybody mind kindly redirecting me to the correct place?

Pretty much, I'm thinking of picking up one of these games some time in the near future, though I have one main question; Will this game run on the old 3DS devices? Will it run at all, or are there any significant FPS issues that make the game completely unplayable?

Few other questions I have (No spoilers please!): are these games' stories different from Sun/Moon, or is it more of a "third game" situation where it's just the same thing with some extra content here and there?
Also, where can I find a spoiler-free list of all the major differences between Ultra Sun and Ultra Moon?

Thanks in advance.

This game will run on a regular 3DS as well as Sun and Moon did, meaning: the performance won't be optimal at times, but the game will remain enjoyable.

The stories... well, it's more or less a third version situation: some of the earlier cutscenes were rewritten to be shorter and, thankfully, less frequent, but aside from a few differences in progression here and there, the story remains mostly the same, until a point towards the end where it completely goes off the rails. The post-game quest, on the other hand, is completely different.

I would suggest to just hop onto the USUM train, tbvh I feel USUM is an DLC to SuMo, albeit altered in very little things and details so unless you're particular on not missing anything Pokemon-ish.

I feel like the game is a lot better optimized than vanilla SM as a whole, but doubles still run pretty bad, especially on the original 3DS. Totem battles on the other hand run a lot better.

For the story, it's actually significantly changed. The story in itself is the same, but they restructured the plot, and the characterization is different. There's a few awkward moments as a result, but the writing and characters (imo) are significantly better. A lot of this is subtle though until you reach the Aether stuff.