As you may know, by using the following url, you can \'ping\' google to update its record of your sitemap:
https://www.google.com/ping?sitemap=URLOFSITEMAP.x
Take a look at the code for the Google Sitemap Generator Plugin for Wordpress. A lot of people use that and it does not seem to slow things down enough to notice. Whatever methodology they are using should be a good place to start.