I'm not sure 25,000 lines translated in 2 weeks is "fast", for a naive translation between languages as similar as C++ and Rust (and Ladybird does modern RAII smart-pointer-y C++ which is VERY similar to Rust). You should easily be able to do 2000+ lines/day chunks.
Agreed, however, I'm quite sure 25,000 lines translated in "multiple months" is very "slow", for a naive translation between languages as similar as C++ and Rust.
Yeah, it also a lot that the person doing the translation is the lead developer of the project who is very familiar with the original version.
I imagine LLMs do help quite a bit for these language translation tasks though. Language translation (both human and programming) is one of the things they seem to be best at.