Accessing the spatial index of a layer (shapefile) with PyQGIS

Question

I built a manual index on a shapefile, through the process "Create spatial index", which can be found in the process toolbox > general vector tools. This process creates a persistent spatial index, a .qix file.

I want to optimize the query: "How many polygons of a layer are contained within one or more larger polygons that I select" using PyQGIS.

Both layers are based on files in polygon shapefile format. The layer of the polygons that will be evaluated has more than 300.000 entities.

I can build and use a custom index with this code:

pry=QgsProject.instance()
parcelas=pry.mapLayersByName('parcelas')[0]
radios=pry.mapLayersByName('radios_censales')[0]

tiempo_inicial = time.time()
index = QgsSpatialIndex() # Spatial index
index = QgsSpatialIndex(parcelas.getFeatures())
duracionSI=time.time()-tiempo_inicial  #  6.816403388977051 sec

csi=0
lisIds=[]
tiempo_inicial = time.time()
for i in radios.getSelectedFeatures():
   bbx= i.geometry().boundingBox()
   listIds=index.intersects(bbx)
   for j in listIds:
        if i.geometry().contains(parcelas.getFeature(j).geometry()):
            csi=csi+1
duracionSI=time.time()-tiempo_inicial

But what I really want to do is leverage the existing .qix file directly, so I don't have to spend six seconds compiling the index every time. How can I do this from PyQGIS?

Note: I am aware that the spatial index file must be updated if geometries are modified, removed or added.