They optimized the first one with Enemy Within, and the DLC packs. Honestly, I never really felt like the first game was all that unoptimized anyways. Granted, my rig's fairly beefy, so it could just be relative
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And...no. The second one is PC exclusive, and there's a lot of things they've taken away (while adding visceral content). Lots of Quality of Life improvements, like consolidation of buttons, expansion of classes( ie, there's literally no "garbage classes". Looking at you, Heavy.), and removal of arbitrary gaminess(by arbitrary gaminess, I mean, in the first game, come on. XCOM is a globally funded paramilitary group. You should be able to field a squad for every goddamn abduction site. Anything less is complete, contrived BS.) However, in XCOM 2, you're playing as a mobile guerrilla force, but you can only be in one place at a time due to your nomadic nature. THAT makes sense. There's also a hell of a lot of depth added. For example, with the environment being more destructible, your tactics can now include things like - taking out the structural integrity right beneath your opponent's feet, forcing them to either fall to their death or regroup. Most notably, if this doesn't kill them, it removes their current overwatch. Also, the game is more than simple move and overwatch now. You could literally beat the entirety of EU/EW just doing that. As most missions have timers in place (and ones that make sense if you pay attention to all of your comms), your tactics actually have to change on the fly.
Things I've noticed that new players are complaining about:
- The new aliens are too smart. (This is a problem between the user and the screen, sir).
- Hit chances don't seem to reflect actual outcomes (...is this your first XCOM?)
- "I keep having to kill a mind controlled squaddie!" (Have you tried killing the Sectoid? That works 100% of the time.)
Anyways, after 28 hours, I'm hooked. Haven't won a campaign yet, but now that I've gotten a grasp of the Advent Doom Counter (and how to counter it), I'm sure I'll beat it at least once soon 
Oh, some things I've learned that might help:
- EXO + Ranger is awesome. You sacrifice a tiny bit of mobility, but due to the nature of how the Ranger plays (putting itself in danger a LOT), it will start paying for itself soon enough.
- Grenadiers may have that extra slot for another grenade on paper, but I've found it to be more worthwhile to equip them with a nanoweave vest or a Mindshield in that slot.
- Sharpshooters actually benefit more from a stock than from a scope. WHO KNEW?
- When there's an opportunity to use the environment to your advantage, do it now, before the enemy can miss you, but hit it and melt your squad.