Not a single developer. The game itself is a product of a Factorio factory, the developer simply drags belts around and flips logic switches to produce new features. The automation has reached a point where it has developed Copilot-like AI skills and very little needs to be done by hand now.

Mistake. The Factorio team is much bigger than a single developer https://www.factorio.com/game/about

And multiple of those devs have posted blog updates about various optimizations they've done. So performance isn't one person either.

Initially it was one developer but it hasn't been just him for quite a while. Nowadays they are a team of more than a dozen I think.

They have poured a lot of work into making their game run efficiently. No matter how much they do that, players will always expand their factory until the CPU cannot keep up.

They have nine as of 2021. https://factorio.com/blog/post/fff-366

They even have a chart of lines of code in Factorio over the years.

it is a reasonably small team but definitely more than one developer

https://www.factorio.com/game/about

There's more than one person but it's a small team, core development on the pre-1.0 versions was done by three or so people I believe.

Just a tyop.