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Ok, what's the underlying storage?

High speed cameras usually record into RAM and then dump the recording out to an SSD in slower-than-realtime when it ends. Since the recording duration is limited by RAM capacity they have rolling modes which record continuously into a ring buffer and then store the current contents when the shutter is pressed, so that unpredictable events can be captured after they happen.

Is RAM even fast enough? I remember reading somewhere that the high speed cameras have special cache-like memory on or near the processor that is faster write than RAM. I can't find it now though so I'm taking it with a grain of salt.

Maybe in specialty cameras with ultra low maximum recording durations but it's usually just a bunch of DRAM, take this Phantom camera as an example.

https://www.phantomhighspeed.com/products/cameras/tmx/7510

The spec sheet lists up to 512GB of RAM depending on configuration. There's no way you're scaling cache-like SRAM up to those numbers.

The necessary data rate for these photo finish cameras isn't actually that high, because every "frame" is only 1 pixel wide. If the photo is 10000 pixels high, you need a datarate of about 10 Gbit/s at 40000 fps and 24 bits/pixel. RAM is plenty fast enough for that.

Regular high-speed cameras that shoot a video are a whole other story, though.

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