They did say in Las Vegas that they've tweaked their lighting engine a bit, but that's really all that was said. Nobody knows to what extent and if anything else has changed.

To be fair though, the new system requirements are very inline with what I'd expect for any game in 2017, and I'm not surprised. That said, my girlfriend's laptop with an i7-4700MQ and a 740M can run Heavensward on Low with around 25-30 FPS, which is fine. I've done EX farms myself on that computer without any issues. If they do make the game more demanding, it'll definitely struggle... but it is over three years old.

Tips for those of you who struggle on low settings: Definitely run the DX9 mode (the effects in DX11 that can't be turned off make the improved threading a moot point for weaker systems), and make doubly sure that Shadow Cascading and Glare are turned off. The game looks like crap without those settings enabled, but you'll gain another 5-10 FPS.