When combined, 长篇大论 (cháng piān dà lùn) literally translates to “long piece of writing, big discourse.” This paints a clear picture of a speech or text that is grand in scale and length, which has evolved to imply it's unnecessarily so.
In many aspects of Chinese culture, efficiency and clarity are highly valued. A discussion should ideally be concise and to the point (言简意赅 yán jiǎn yì gāi). To subject others to a 长篇大论 is often seen as disrespectful of their time and intelligence. It can imply that the speaker is self-important, poorly prepared, or lacks the ability to distill their thoughts into a coherent message. A useful Western comparison is the concept of “being lectured” or someone “getting on their soapbox.” While a “lecture” can be neutral, being “lectured at” is always negative, just like a 长篇大论. It's more critical than simply calling something a “long speech.” It implies a power dynamic (e.g., boss to employee, parent to child) where the listener is forced to endure the rambling. It highlights a cultural preference for communication that is direct and considerate, rather than indulgent and verbose.
长篇大论 is a very common idiom used in daily life. Its connotation is almost always negative or, at best, sarcastic.