An engineering team is deploying a new text-generation service that handles a wide variety of user requests, from single-sentence completions to multi-page document summaries. They initially implement a simple round-robin load balancing strategy, which sends each incoming request to the next available processing unit in a sequence. Despite having ample processing capacity, they observe that some units are frequently idle while others have long queues of pending tasks. Explain why the round-robin strategy is performing poorly in this specific scenario.