My guess is that one of the fundamental differences there would be that KMZ isn't streamable and needs to be fully loaded into memory and then transformed into

My guess is that one of the fundamental differences there would be that KMZ isn't streamable and needs to be fully loaded into memory and then transformed into whatever structure qgis uses internally, but I'm not totally sure about that and haven't used QGIS in a minute. I feel like I've also had bad luck loading KMZ/KML of any reasonable complexity into any other GIS app. How's geojson of the same data?