MacOS/Macbook Pro (Mid 2015)
So, with retina screens the pixels are 4x as dense, which means that most software (specifically those based off of a vector/non-image UI) need to render 4x as many pixels to show on your screen without looking clunky and pixelated. The problem with this is that it takes up more processing than needed. Most of us retina users would normally go to: [Wherever Blender is Located>Blender>Get Info>Open In Low Resolution] to open the app in a normal resolution, which lets us use blender without having crippling lag in the viewport. Well, for some reason checking the "Open in Low Resolution" box does nothing, and, although the UI is super sharp, it has immense lag and input delay from the excessive amount of pixels it has to render.
Exact steps for others to reproduce the error