The problem isn’t so much one of accurate measurement (although that IS very much a thing, because by measuring you disturb the electron and change it’s momentum), but we are looking at the quantum realm here. And for quantum particles, it’s not just impossible to measure speed and position to an arbitrary position, but there is a lower level to which those even physically exist.
Where classical physics saw an electron more as a point mass, in quantum physics an electron is better represented as a wave packet like here: https://eng-web1.eng.famu.fsu.edu/~dommelen/quansup/packetm0.gif
The thing is with a wave packet like that, the speed is defined by the shape of the wave more or less, but… The more stretched out the wave is, the less you know where the electron is, but the more you know about its speed. If the wave packet is more contracted, more like a point, then we know better where it is, but the velocity is less defined.
So this lower limit is not because of some flaw in our calculations or measurements, but a fundamental property of quantum particles.
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