Slow Mo Guys have a Phantom that rocks up around 1.5 million FPS. They've used it to show how glass cracks etc. when you get to that speed though, you need a LOT of light, and a very low resolution.
i dont imagine the low resolution is per se essential the same way the light is. you could maybe figure out ways of syncing the data collection to match each other at that high of speed
It is because the main limitation for high speed digital video is the bandwidth of the pipeline from sensor to storage. Lowering the resolution increases the number of frames you can shove through the X Gbps pipe between the photons hitting the sensor and the highspeed memory the camera is buffering into.