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.
Rust and C are the same “tier” of performance, but GNU coreutils has the benefit of several decades of development and optimization that the Rust one needs to catch up with.