xmlsitemap_node_get_timestamps() maintains a static list of timestamps for every node it has seen in the current request:
<?php
static $timestamps = array();
if (!isset($timestamps[$node->nid])) {
...
?>When creating thousands of nodes in a process (say, hypothetically;), when migrating), that can add up. Using drupal_static() for this static cache at least presents the opportunity to recover the memory. The Migrate module does this when it senses memory_limit is drawing near, and generally recovers plenty to continue as long as no hooks are using local statics like this.