Probably the same way a hybrid gas-electric car is more fuel efficient. In a hybrid, the battery revs up and down with need while the engine just powers the battery at a steady clip. Since the engine can run constantly at a fix optimum speed, it is more fuel efficient.
Likewise, I figure, each server has a certain amount of bandwidth. If everyone is on one server, all the posts and comments come at random intervals with spikes and troughs. Either the bandwidth gets throttled, which causes lag, or all the comments go through at the same time, which uses a lot of bandwidth. With multiple servers, those posts get federated and (probably, I’m guessing at this point) wait for the federated server to signal that they are no longer busy, which flattens the bandwidth demand.
Imagine the intelligence and attention-span of your average Anime fan. Now realize half of anime fans have lower levels of attention and intelligence.