> My impression is that it's memory-limited, and it has a builtin limit.

Factorio is primarily bound by memory bandwidth, believe it or not. You need a healthy amount of memory for really large factories, but the limiting factor ends up being the fact that a significant amount of it gets accessed every frame.

There is no built-in limit on factory size.

> Anyway, your factory size is FPS-bound

Not at all. FPS is only influenced by the amount of stuff on screen, which is bounded by the size of your screen. The most expensive scenes tend to be dense forests, not factories. :)

> Not at all. FPS is only influenced by the amount of stuff on screen, which is bounded by the size of your screen. The most expensive scenes tend to be dense forests, not factories. :)

In factorio frames aren't interpolated, so always FPS ≤ UPS. This means that while you're not technically limited by the rendering speed the FPS does bound the factory size.

Note that is a memory speed bottleneck, not a memory capacity bottleneck

My best guess is that the game misses some optimizations due to enforcing a limit on memory usage.