I understand the basics of spatial indexes but would like to understand when exactly a spatial index would need to be re-created or updated?
The reason I ask is because when a folder was moved to a new drive on my computer rendering slowed down considerably for the shapefiles contained within it. Before moving they all had a spatial index created. After the move and after I realised the rendering had slowed I created new ones not really knowing if it would help but it seemed to work.
I use QGIS 2.18.20