Before there were shards, the MMO was just shard 0. As it was the only map/server it was even larger than it is now, and tick times (If I recall correctly) even longer.
With the advent of shards, S1 started out small and opened up more but still stayed a smaller size, S2 followed and finally S3 which is CPU restricted and was billed as the non subscriber shard, where everyone was on 20 CPU across the board.
S0 since then, has had rooms slowly being deactivated in order (I assume) To speed up tick time, including the time when a whole out-side ring was collapsed:
There were/are some player movements who went around and tried to take out zombie players and clean out S0 rooms, but not sure where that stands now.
S0 also hosts some of the older players, their codebases. Not sure the exact factors that slow it down, but considering it has 11339 claimable rooms and no CPU cap (so older players have at max, 300) While Say, S3 has 7076 rooms and a 20 CPU cap on everyone, it prob slows it down a bit, even though S3 has way more players.