It's funny that lots of people comment on the snappy UI, because there isn't actually a lot of performance magic going on here -- the frontend just loads a few-MB large gzipped JSON inverted index and uses it to do all search and display locally in the browser using pretty normal unoptimized JavaScript. No WebAssembly or anything else fancy -- just JavaScript working through a large object graph.

I think in general simple JavaScript applications can be remarkably fast these days if you're careful not to introduce unnecessary work and aware of the data structures you're using and profile + optimize judiciously :)