This is bit late but here is what worked for me…
During the insert process there are a few WordPress variables that get quite bloated, so once you have inserted say, a 1000 posts, you can reset them:
global $wpdb, $wp_actions;
$wpdb->queries = array();
$wp_actions = array();
wp_cache_flush();
These were the main culprits in my case…and releasing them freed a good chunk of memory
In my case for a bulk insert of 6000~ posts it was originally taking upto 300 MB, but after releasing the memory after every 1000 inserted posts memory usage never went above 50 MB.
ps. You can use var_dump(get_defined_vars()); to have a look at whats taking up memory in your case and free variables appropriately.
