Gsitemap module is great and I have been using it for a long time. It can notify the search engines whenever your sitemap changes such as when a new node is added. However, if you select this "submit map when updated" checkbox, then that can degrade performance. I had turned it ON once and noticed that the submit new node was taking 90 seconds. When I turned it OFF, it became 7 seconds. Performance is slow because the extra work of notifying search engines is being done - not a fault of the gsitemap module. I hope this finding helps others facing performance problems only in submit of new node.