1. There is no video lag with a proper setup.
2. Proper setup is native resolution (e.g. 4K @ 3840x2160) or HiDPI (= retina, = pixel doubling) scaled resolution (1:2, or 1920x1080 on a 4K screen). It is fairly inexpensive for the GPU to render 3 x 4K screens with these settings.
3. Improper setup is any other fractional (non-integer) scaling. Why? Because depending on monitor (retina or not) and setting, you will either:
- have blur due to interpolation when rendering non-existent thirds and halves of a pixel
- tax the GPU by using a virtual 5K internal rendering, which is then down-scaled with proper pixel-doubling
In worst case, both will apply.
4. If you are a photographer, artist or use the computer for anything visual other than text, you will frankly not like blur and fractional rendering. You want your pixels to be true representations. You want native resolution or pixel-doubled integer scaling.
5. If you are indeed a text man writing letters, you will be fine, but then this computer is probably not for you anyway.
6. If you think it is a ”user preference” to go against trivial phsyics (as so many here seem to do), then you do need an eGPU (or another computer), but remember that fractional scaling will always result in some lag no matter what GPU you have. Because it is using floating point arithmetic for eight million pixels sixty times a second, or half a billion operations just to calculate how to present the screen. That's not what GPUs are intended for, especially not an Apple eGPU that costs $800 for a $200 card that can't be replaced.
7. The GPU uses 1.5 GB of the RAM, so with 8 GB installed, you effectively have 6.5 GB. Some seem to think this is just fine, but it will result in some lag when the system starts to compress and swap. If you indeed use PS and AI you will be happier the more RAM you put in. Next time you import a batch of RAW photos from your camera, have a look what happens with memory in the Activity monitor, and you will understand.