When are slow songs always in minor keys?

Slow songs are not always in minor keys; they can be composed in major keys as well. The emotional impact of a song, whether slow or fast, often depends on various elements like melody, harmony, and lyrical content. While minor keys typically evoke feelings of sadness or introspection, major keys can convey warmth and happiness even in slower tempos. Ultimately, the choice of key is a creative decision made by the composer.