I have long suspected the shared RAM of integrated GPUs causes memory contention and significantly slows the performance of the CPU. Especially in the context of compiler and IDE performance.
Have you done any experiments or noticed a difference when adding or removing a discrete graphics card?
Are you aware of any studies on this subject? (I could not find any)
For video there's 2 uses of memory - reading the frame buffer's contents and sending it to the monitor every frame; and whatever the GPU happens to be doing.
For the GPU there's no way to guess.
For reading the frame buffer; for a video mode like 1920x1600 with 32 bits per pixel you're looking at 12.288 MB per frame, so at 60 frames per second that's 0.737 GB/s. A single RAM module is typically capable of "tens of GB per second" (e.g. DDR4-3200 is 25.6 GB/s according to wikipedia). From this you can assume reading from the framebuffer consumes less than 10% of one RAM module's bandwidth. Of course for most systems there's multiple RAM modules and multiple memory channels; so it's likely to be significantly less than 10% of available RAM bandwidth.
Also note that CPUs typically use caches for most memory accesses and only need RAM bandwidth for "cache miss" (e.g. you could have 8 CPUs pounding caches and still have almost all of the usable RAM bandwidth wasted/being used for nothing); so devices of all types (e.g. disk controllers, network cards, USB controllers, sound cards, discrete and integrated video) using RAM bandwidth won't necessarily effect CPU performance.
There are also other (potentially more significant) factors for performance too. For example, for modern integrated video, GPU is in the same package as the CPUs, so when the GPU is going berserk heating up the package the CPUs may need to slow down to avoid melting everything. Discrete video cards don't have this problem (they have the "spend several hundred extra $$ to be deafened by excessive fan noise while you're sitting in a puddle of your own perspiration" problem instead ;) ).
Mostly; everything involved (which hardware, which software, which other devices) is too variable for a concrete measurement of one specific case to be meaningful; so I wouldn't expect to find any studies.
