You quoted: "Learning a language is an amazing feat-one that has attracted the attention of linguists and psychologists for generations." "Feat-one" is not a "phrase." It is not a hyphenated compound word. Due to clumsy typesetting or a typographical error, a dash, which is a long line, has been replaced by a hyphen, which is a short line. A dash is a kind of long pause. In print, it is technically an "em-dash" and is one "em" long. The sentence should be: "Learning a language is an amazing feat—one that has attracted the attention of linguists and psychologists for generations." Can you see the difference between "feat-one" and "feat—one?" It means: "Learning a language is an amazing feat. [It is] one that has attracted the attention of linguists and psychologists for generations" Hyphens are sometimes used to link two words into a compound word, like "mother-in-law" or "well-being." However, this is not a compound word. "Feat" and "one" are not being linked.

be slow

No clue. Context?