I've created a Spatialite layer with all of the waterbodies and watercourses in Saskatchewan for use in QField. Rendering is quite slow on both my older laptop and my field device - a G5 cell phone. When I look at the layer properties I see that the "create spatial index" button is disabled.
I know that Spatialite supports spatial indexes, but it's not clear from what I've read whether the newer virtual spatial index is created automatically or if I need to enable it somehow. If it is truly virtual then I assume it is created on the fly, which would be a bad thing in the case of QField. It's also not clear whether QField will use spatial indexes but that's a question for a different forum.