I don't see anything obvious, other than the lag.
Three more troubleshooting steps I could suggest:
1. Run the posts query to see how long it's taking. That's all that is in the edit.php- it did a query, then formatted it with the options, i.e. select * from wp_posts where post_type = "page"
2. Create a plugin script, and try to replicate what the php script is doing in creating the posts by query, i.e.
$querystr = "SELECT * FROM $wpdb->posts as wpost ORDER BY wpost.post_date DESC";
$pageposts = $wpdb->get_results($querystr, OBJECT);
3. Then in your plugin, change it so that it's getting them by using the wp_query engine, i.e.
<?php $args = array(
'sort_order' => 'asc',
'sort_column' => 'post_title',
'hierarchical' => 1,
'exclude' => '',
'include' => '',
'meta_key' => '',
'meta_value' => '',
'authors' => '',
'child_of' => 0,
'parent' => -1,
'exclude_tree' => '',
'number' => '',
'offset' => 0,
'post_type' => 'page',
'post_status' => 'publish'
$pages = get_pages($args);
4. Last, create the site from scratch, do the import, and see if you get the same problem.
I really do think it's something with database performance, and you can't check that kind of stuff from the wp admin.
Sorry I wasn't of more help.