[Cthulhu_]

Well, at the smallest level (if I recall correctly) it's a single solar system; they have a few beefy servers on stand-by, one is always in use for Jita, the main and most active trading hub, and others can be spun up and a whole solar system transferred over if it gets busy. And as someone else pointed out, you can organize a large fight in advance so they can transfer it over.

But there's the bottleneck, because they can only do the calculations of ship movement & actions on a single node. I'm sure it's been optimized to no end as well. IIRC it's written in Python, but that's not going to be the main performance bottleneck.

I have only the smallest of clues about distributed systems, the only way they could scale it up is to somehow make it so they can run a single solar system or cluster of ships on multiple servers, but for that you get the overhead of inter-server communication or you need an asynchronous, eventually-consistent game instead of something realtime.