I've only got about 5 hours into Unity, but I haven't noticed any problems with frame rate or anything like that. I have noticed a few more NPCs walking about in trees, but nothing worse than the previous AC games. But, then, the only games I have on my PS4 are AC: Unity and Middle Earth: Shadow of Mordor, so I'm not sure if having that extra space on the hard drive makes any difference at all.
If you're starting the franchise totally new, I'd say to pick the time/place in history that most interests you. The framing story is nicely recapped at the start of every game, so that's not a big issue. History is AC's playground--play where you'll enjoy first, and then, if you like the game play and the story, look at the other games. Some you'll like better than others, likely. It's all good.
Myself, I'm enjoying Unity thus far, though I'm not used to the stealth button stuff (I don't play a lot of games), so I need to practice that. A lot. Especially before I start going for co-op.
I've also decided that I'm not dealing with the app stuff or any of that bull. I don't own a smart phone and I'm not buying one for game-tie-ins. Ubisoft can bite that as far as I'm concerned. Anything that doesn't come with what I've already paid for, I'll just deal without. And if the game centers too much on that stuff, then they've lost a long-time pre-order customer who's not shy about explaining why the game started to suck Gibraltar through a bar straw, y'know? They already lost my money with Rogue, after all.