The main thing to realise is that not everything has to go into the ECS. This is a bit more clear with middleware like physics engines where you’re forced to in

The main thing to realise is that not everything has to go into the ECS. This is a bit more clear with middleware like physics engines where you’re forced to integrate two views of the world. The game view and the physics view and need to translate and update between the two. Similarly with data in an ECS you can separately process spatial relationships either reusing the required data or keeping local copies as an optimisation.