So far I haven't ran into performance issues, but this version doesn't have networking, so I only tried spawning lots of monsters in a small map, so they have to recalculate their movement quite often + each monster runs an AI loop for its actions, didn't drop an FPS in my M1 Mac that I'm using for development, which is quite beefy in terms of computing power. Anyone can retry this simulation. Open the demo, load "Shop" map for example, which is small, and spawn lots of monsters, see how it performs on your hardware.
One thing to note that, at least in terms of rendering, monsters are far simpler to draw, since they originate from a single sprite. Players, at least in in the multiplayer setting, have each equipment sprite drawn separately, which means that having lots of player characters in a single screen can have higher performance penalty, but this option cannot be simulated with the current version.