QGraphicsScene applies an indexing algorithm to the scene, to speed up item discovery functions like items() and itemAt(). Indexing is most efficient for static scenes (i.e., where items don't move around). For dynamic scenes, or scenes with many animated items, the index bookkeeping can outweight the fast lookup speeds.
For the common case, the default index method BspTreeIndex works fine. If your scene uses many animations and you are experiencing slowness, you can disable indexing by calling setItemIndexMethod(NoIndex).
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