I've previously helped to develop http://opac.provincia.bergamo.it
, which is using Hibernate Search 3.0.1: rather old, as it was some years ago.
There's a public search page, so you can try some queries. It's usually very fast, when you see a slow query, that's because the engine decided to perform a relational query ;-)
You can find some information on the hardware running the website and the overall technology stack here: http://www.jbossworld.com/2008/jbia/alintec/
That's just a small example but the performace is great, as it's one single relatively cheap server and serving a large amount of searches every day.
I know of far larger deployments on websites of well known brands but unfotunately we're not allowed to disclose them.
You can find a lot of references online about Apache Lucene, which is the same search engine that Hibernate Search uses; we strive to make it simpler to use and provide a nice integration with Hibernate, but in terms of features and performance it's the same thing.