Throughput: the number of frames displayed per unit of time.
Latency: the time it takes to capture, process, and display a single frame.
Refresh: the rate at which the display 'paints' frames (which is tied to the underlying mechanics of the display and keeping display elements refreshed).
You could theoretically have 120 fps refresh and 1 fps throughput with a 5 second latency. The display will just paint the same frame 120x, and it will just take 5 seconds to see anything when you first activate the system. The EVF refresh rate is no guarantee of either throughput or latency. If I had to guess...
- Most EVFs can probably achieve a throughput equal to their advertised refresh in daylight when the camera is not shooting.
- Probably all of them experience a severe drop in throughput in low light.
- Probably no EVFs have a latency equal to their advertised refresh.
- Taking a photograph...especially with a mechanical shutter...introduces a very significant 'bubble' into this pipeline. How the system handles this bubble will depend on latency. But if you see an image instead of a black out then it involves using one or more frames outside the normal latency, and using them for multiple refreshes until the system catches up.
#4 is likely the source of most complaints, including complaints of seeing something like a bird near center of frame when it's really out on the edge. During burst shooting the system just falls further and further behind until you stop shooting and it can resume it's normal processing pipeline. But #3 can be detectable with rapid movement and scene changes. And #2 cannot be ignored for low light shooters.