Tests have been performed with a lot of data in the Search app. The result was that it still performed well. We assume that you do not offer all documents in the app, but that you make a conscious selection of the documents that you really need online.
When you are online, the app first fetches the collections. Only the moment you open a collection will it retreive relevant documents. If you then click on a document, the document itself will be retrieved.
It can become more data when used offline. The data must then be put offline by the app, so that can cause a difference in performance.