For most Highspeed-Cameras it works like this:
The Sensor COULD put out a LOT of FPS, just the Electronics in the Camera Limit its performance somewhat.
With top of the Line - Highest Priced Cameras i guess the try to use the Full Potential of the Sensor, but for something like the chronos, i think the Developement was going more for overall reasonable Performance at an (for the normal Human) somewhat affordable Price Point (i actually feared, that an 1080p Chronos would be more Like 10k or above in price!). That is, why they propably where not able to use The Full Sensor FPS Capeability, because Fast Electronics, which need to process this Data From the Sensor could get Very Expensive very fast!
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Because of Max Framerate:
Just Look at something like an Flex 4k and those other Phantoms, that do crazy high FPS at 720p Full Sensor Resolution.
It is quite common, that cameras, that have higher Res. Sensors will be a bit Slower.
I am No expert on this, but keep in Mind, that, even if you dont use it (for Lower res. faster Speed), all the Pixels from the Sensor still are there and the Electronics in the Camera still have a lot more data to process, even if you just use 1920x8.
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About the weird 1920x8:
Also not sure about this one, but for some cameras (i guess it is the same thing with the 2.1 Chronos) Max FPS won´t chance if you change Horizontal Resolution. That means, your 125kFPS Max will be the Same, no matter if you record in 1920x8 or in 8x8.
That is, because the data is processed in Lines, not on pixel-by-pixel base. If someone knows this better, feel free to correct me, but i think it works this way. Only benefit of going 8x8 instead of 1920x8 would be, that you should be able to Record MUCH longer with 8x8, because the Lines would still be processed, but only 8x8 pixels would be saved into memory.
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So, it could actually be possible, that you will be able to get more FPS out of the 1.4 than the 2.1, but pushing insane FPS does not look to be main priority with the 2.1, it is more about Image Quality. And that is a very good thing.
Maybe the Kron guys will come up with an crazy high Framerate model of an chronos at some point later, but i think for now, it is a very wise next step from them to go with maximum image quality.