I have been watching a TV show about cheetahs, and the narrator used the following sentence:
Speed is key,
As far as I know, "key" is neither abstract nor uncountable. I don't understand why 'the' is omitted before 'key'.
key can be an adjective (of paramount importance, necessary, critical, crucial) or a noun.
"Speed is key!"
If you googled the phrase, you'd know that its author is an Irish producer, game commentator, and internet personality known under the pseudonym of Jacksepticeye. In one of his videos he uses the phrase when going fast or wanting to go fast in a computer game. And of course, there, the word "key", grammar-wise, is neither abstract nor uncountable noun – it's an adjective already defined by the previous answerer.
Actually, "Speed is key" is a meme which is a cultural unit/idea/pattern of behavior that is passed from one person to another by non-genetic means (as by imitation).
An additional source is here.
At the same time, if a key is also a part to be pressed by a finger that serves as one unit of a keyboard (and there may be a special key designed for the acceleration in a game), the above-mentioned person, speaking too fast, might just have skipped the definite article. You can never tell with memes.