Spacial Partitioning - Quad Trees
An important element needed for various things in the engine are Quad Trees.
Here is an example of a Rectangle checking collisions against random pixels on a bitmap data.
Quad tree
As you see with a quad tree it can easily handle 100'000 objects and do collision detection. If it wasn't for the long loading time it could even be a million or more.
The quad tree in the example isn't fully optimized and visualisation slows down performance of course.
Here is another example testing rectangles vs. a rectangle:
Quad tree 2