I mean, obviously using different languages has different trade offs in many dimensions. If speed were all that mattered then everything would be written in ass

I mean, obviously using different languages has different trade offs in many dimensions. If speed were all that mattered then everything would be written in assembler. But the point at issue is that by using java you are trading off speed (potentially for other benefits). By the way - tantivy at least is implemented in rust so it shouldn’t suffer from memory safety issues just as lucene shouldn’t. And the rust language will provide some guarantees that java doesn’t - like freedom from data races.