When comets fly by Earth, where we can easily see them in the sky, they are always going very quickly compared to speeds we are used to in daily life. The comet NEOWISE that you might have seen earlier in 2020 was flying at 144,000 miles per hour - if you could travel at that speed, you would be able to get from Santa Barbara to New York in just 1 minute!
The reason the comet does not look like it's moving is because it is very far from Earth. Far away objects will always look like they are going very slowly, or even not moving at all. Think of how an airplane that is taking off or landing near you looks like it's going pretty quickly, while one that is far up in the sky seems to take a long time to go anywhere. The airplane far up in the sky is actually going even faster than the one nearby, but because it's so far away, it looks like it's slow. The comet is so far from Earth that even though it is flying very fast, to us it looks as if it is almost not moving at all.
Astronomers can measure how fast the comet really is, and how far away it is, by watching and writing down exactly where the comet is in the sky over a few days or weeks. Gradually, it does move relative to the stars we are used to seeing. With this data, astronomers can figure out where the comet actually is in our solar system, and where it will go next.
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