Here's why:
* Common nouns refer to general, non-specific things. They are not capitalized unless they start a sentence.
* "Speed" is a general term that refers to the rate at which something moves. It's not a specific person, place, or thing.
For example:
* The car's speed was impressive. (General term, not specific car)
* I need to reduce my speed. (General term, not specific person)
Let me know if you have any more questions about nouns!