We haven't forgotten, we choose to put our time and energy into features rather than performance because that's what end users care more about (at least, it's what they vote with their feet for).
The difference can be felt when comparing a game like Super Mario Bros running on an emulator with a typical LCD screen to running on original hardware with a CRT monitor. The emulated version is so sluggish it feels like you're playing underwater!
The techniques haven't been lost - if anything there are probably more people who know how to do those things today than there were in the past.
Typical LCD monitors are undeniably worse in many ways than the CRTs they replaced, but again, most customers turned out to prefer the convenience of the LCD on the whole. Nowadays with the rise of 144Hz monitors those rare users who care a lot about gaming latency do have an option.
I would expect the NES to perform similarly to the Apple, since it has the same CPU (6502) and also uses a CRT display.
I as a freelance developer sometimes make releases whose sole purpose is to refactor/slim down/increase performance with no new features. Yeah, some of my users are disappointed I didn't prioritize X feature they wanted, but it helps mitigate tragedy of the commons a little.
Individually the users are well-meaning with their feature wants/needs, but collectively all the different feature requests can turn your app into a bloated monstrosity if you aren't careful.