Bartek contributed to the typelevel/cats repository by optimizing the distinctBy method for NonEmptyList, NonEmptySeq, and NonEmptyVector collections. He refactored the implementation to use iterator-based traversal and a mutable TreeSet, reducing memory allocations and improving throughput for large datasets. His work preserved the public API while enhancing internal performance, and addressed edge cases such as single-element collections. Bartek ensured compatibility with older Scala versions by using lengthCompare and improved code clarity by refining implicit Ordering scoping. Throughout, he applied his expertise in Scala, data structures, and functional programming to deliver maintainable, performance-focused improvements to a widely used library.