Function score 

pub fn score(
    index: &[Arc<[Entry]>],
    query: &str,
    base: Option<&Matches>,
    limit: usize,
) -> Matches

Score query against the files in index, keeping the best limit to list.

With base from a prefix of query, only its matches and the files indexed since are looked at. Called off the UI thread: over a large tree this is the work that made every keystroke wait.