In Notes 10 there is a new feature which will automatically run FT index. This will run if some documents have been added since last time the index was updated. The effect of this to the user can be a noticable delay when perfroming a search as the index will run first.
It could be added to the functionality to allow the user to specify at what threshold the auto index should run (e.g. they could specify that if 200 documents or more have been added to the index that the FT index should be automatically run. Also it could be added that the search would first search the existing indexed documents before then running the auto index on the newly added documents. This would at least provide something quickly to the user so they do not have to wait with no returns.