Here I have to speculate because I just don’t know enough about the technical side of it. At the moment, most issues seem to be cause by software bugs, not by too much traffic or hardware performance.
Handling high amounts of traffic and activity is always tricky. I believe scalability will probably be an issue that will arise, maybe sooner than later, but I don’t think it’s an unsolvable issue.
I'm running a small instance with around 55 communities/magazines. But, running on kbin. I'm currently getting an average of 5 federation messages a second, of course most of them are like notifications. The LA on the server is 1.55 1.35 1.05 right now, that's with a 16 thread cpu, and the server is also doing a lot of other things. At the current level of traffic, the load of federation at least is negligible.
I only have myself as a user right now though, because for whatever reason my instance doesn't show up in fedidb unless you actually search for it. But it's not in any of the lists they publish of instances they are blocking or hiding.