So each frame you must build/extract list of entities based on system?
How do you do that with minimal performance loss?
Good catch, thats the one part I havn't gotten right now. It would be way faster to just iterate through all entities directly since this is what happens in EntitiesWithComponents anyway, so its a double performance loss. I think about caching entities by often used combinations of components, this should speed it up a bit. There are many other possible ways to handle this though, just havn't found the motivation to tackle this, CPU Performance isn't a real problem right now too