Ubuntu 25.10’s transition to using Rust Coreutils in place of GNU Coreutils has uncovered a few performance issues so far with the Rust version being slower than the C-based GNU Coreutils. Fortunately there still are a few weeks to go until Ubuntu 25.10 releases as stable and upstream developers are working to address these performance gaps.
Language isn’t everything. While Rust provides some features and safety that C doesn’t while being roughly equivalent in performance, the algorithms that developers choose will dominate the performance impact on the program.
GNU core utils has decades of accumulated knowledge and optimisation that results in the speed it has. The Rust core utils should in theory be able to achieve equivalent performance, but differences in the implementation choices between one and another, or even something as simple as not having prioritised speed yet and still focusing on correctness could explain the differences that are being reported.