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It's a microcontroller. It has maybe 512k of storage and 128k of memory. The Numpy package probably is ~100Mb

The controller is running CircuitPython which is a fork of MicroPython - a tiny re-implementation Python for microcontrollers.

Valid point! Might still be able to get a speed up if you can write and import your own C code though.

The best usb keyboard should have 1ms latency due to USB HID interrupt interval. 4 ms is not that bad.

Yeah, read it as 4ms per row at first. Should be fine for general use, and rather impressive as is :)

How would 4ms be noticeably laggy?

I read it first as 4ms per row, but yeah 4ms per scan wouldn't be too bad. There is of course additional time taken for communication etc, which might take another 4-10ms, which could make it almost a frame behind. Not terrible still.