And that's a naked assertion which goes against observation. People are telling you they can see EVFs fall behind during burst shooting. We'll see how the R5/R6 hold up, but this has long been a problem with mirrorless.
Sony A9II EVF at 120Hz is reported to be very good. I didn't try it myself, but the R5/R6 have even faster readout and CPU and also have 120Hz EVFs.
It most certainly can. The path from exposure to EVF is a pipeline with discrete steps. No single step can be longer than 1/120th (assuming a true 120 fps in sufficient light), but the total path can theoretically be any length of time. That's the difference between throughput and latency.
No it doesn't work like that because it's meaningless, and also requires multithreading. If you have 2 frames being processed at the same time, you need 2 threads, 3 frames - 3 threads etc. It simply doesn't work like that.
With a single thread, the processing MUST be less than 1/120s, it's simple math. You can introduce additional buffers and artificial/unnecessary latency, but why? Just because it's Canon with its cripple hammer?
What heppens in reality, the whole pipeline after the capture is basically readout plus applying some filters (de-mosaicing + current image style), after which it's basically ready for EVF right away. There's no point in breaking it down into many steps separated in time.
It would be detrimental to performance, and difficult to code, if the processor had to interleave reads/writes with display reads. It's far easier to synchronize a single write of a completed frame against the display's 120 Hz read cycle. That EVF has its own discrete display buffer. There's no way DIGIC X is contending with the EVF for access to memory, it would just slaughter performance/efficiency.
Of course it's shared memory/direct memory access by EVF. They do processing in a buffer and the EVF switches to the buffer right away, there's no additional copying/writing anywhere. Basically it takes two alternating buffers.
There's another buffer with overlay (settings/histogram) etc., also used directly by the EVF module, but it takes much less time to maintain.
At the end of the day all that matters is the photographer's experience and performance. There's stutter on the highest end mirrorless bodies. Can you work around it and track any way? Sure. Is OVF blackout less disruptive? A lot of people think so.
I'm more concerned about how the EVF affects the eyes, I've never used EVFs for long periods of time, but even short periods cause discomfort to me. But I'm shooting more and more landscapes and less and less action, especially this year, so I'll mostly be using the LCD screen.
Additional delays during continuous shooting - again the A9II is said to be very good at it, I expect the R5 to be on par. Time will tell.