A quote from this article: https://www.unrealengine.com/en-US/tech-blog/lumen-brings-real-time-global-illumination-to-fortnite-battle-royale-chapter-4
Other quality improvements came by surprise. Lumen GI downsamples heavily in order to fit into a four millisecond budget. This makes noise more noticeable, distracting from the game experience. We were able to reduce noise greatly by integrating NVIDIA’s Spatiotemporal Blue Noise, a technique that pre-optimizes noise patterns to cancel out quickly when accumulated over multiple frames. This gave much cleaner indirect lighting without requiring any more rays to be traced.
This accumulation is reliant on garbage TAA and/or expensive TSR. This is a major issue in the industry that Lumen current perpetuates. Lumen does not resolve this noise on it’s own. That is a major issue and it’s been solved in better ways.