The example (http://lunrjs.com/example/) indexes 100 stackoverflow questions, some of which are relatively long.

If indexing performance starts to become an issue the whole search index can be moved into a web-worker, which prevents indexing from blocking the rest of the page.

Maybe you can create index on the server and then just load it on a client?

edit: looking at the docs it's unclear if it's possible. I guess index should be a JS object, so it's pretty simple to save it to a disc and then fetch it from client.

Since the library can be run outside of the browser (using node.js for example) the index could be generated server side, and then just passed to the client. I hadn't considered this before but it might be worth looking at.