Fair points but still, this is definitely not just overload or it would be felt by all equally? What explains different chars having completely different lag when multiboxing?
Very oversimplified explanation, but:
A very common design pattern is take the total set of all character data and break them up into groups, called shards. Imagine characters beginning with A-L on one server and characters M-Z on a second server. More activity from characters A-L can cause a performance degradation for that shard (database).
This extends further when characters are also broken up by zones and instances within those zones. Maybe the shards get smaller and grow in number, requiring more resources and so on...