Use spatial index to check fewer features in QGIS Actions Buffer example

You can dramatically improve the performance of the https://courses.spatialthoughts.com/qgis-actions.html#select-features-in-a-buffer-zone example by using the option to pass a rectangle to QgsVectorLayer.getFeatures():

line_layer = QgsProject.instance().mapLayer('[% @layer_id %]')
polygon_layer_name = 'buildings'
distance = 20
fid = [% $id %]
line_feature = line_layer.getFeature(fid)
line_buffer = line_feature.geometry().buffer(distance, 5)
polygon_layer = QgsProject.instance().mapLayersByName(polygon_layer_name)[0]
nearby_features = [
    p.id()
    for p in polygon_layer.getFeatures(line_buffer.boundingBox()) 
    if p.geometry().intersects(line_buffer)
]
polygon_layer.selectByIds(nearby_features)

This allows the request to use a spatial index (if exist) to quickly limit the number of features to just the candidates that intersect the rectangle. So the if ... intersects only has to check those remaining buildings. Not much of a noticeable difference in the example data but with bigger datasets it will be a huge speed improvement.

Shout if you'd like it, then I'd make a pull request. Would you like to include both versions for the users to experience and learn?