This is not a hardware issue!!! Both the Intel and NVidia GPU can easily handle both much higher aggregate resolutions than even the 3840x2400 scaled HiDPI mode (Intel has about 20x the required memory bandwidth, Nvidia nearly 80x required memory bandwidth), and can easily easily handle the scaling using any interpolation technique (a very cheap operation). People just seem to repeat what Anand said without understanding whether it was accurate or not:
https://forums.macrumors.com/posts/15526427/
Apple's GPU drivers however are well known to be buggy and clearly slower performance wise than native (NVidia/ATI binary drivers on Windows or Linux for example) equivalents, and the APIs Apple are using are often not even optimised for their poor drivers. Remember how long Apple took to utilise hardware accelerated video playback (years after the hardware was capable and working on Windows, and only for some variants of hardware when all their GPUs were capable)? And that they are incapable of keeping up to date with the OpenGL spec even though OpenGL is the core of Apple's graphics pipeline. Apple's graphics drivers are simply just "good enough".
Again, the fact this affects some people and not others, affects some software more than others, or some versions (i.e. compare Chrome vs nightly Chromium) of the same software, along with the fact that empirically the hardware is more than capable and this should be clear!
