How to cache usernames
As you may know, system function node_load statically caches nodes while called. For example if you look at at node with nid = 1000, system call node_load(array('nid' => 1000)). There is a database query and results are cached in node_load. So, when you call node_load twice for one node, all data comes from cache created on the first node_load run.
