use PostGIS with proper spatial indices, and properly tuned if you have to take the QGIS way. and remember that spatial SQL is always better than using python-b

use PostGIS with proper spatial indices, and properly tuned if you have to take the QGIS way. and remember that spatial SQL is always better than using python-based analysis. particularly with newer versions of Postgre where there is chance that the queries be parallelized.