When you have a large number os posts it’s impossible to create sitemaps by using pseudocron or hooks like save_post. This plugin will require a system cron schedule to visit a secret URL and trigger the sitemap dump procedure. It splits the sitemaps into 50,000 URLs each, and also tries to stabilize the memory usage during the run, doing small queries and using a limited buffer to write the files. If you don’t have administration privileges over your server, you can use a cron job provider on the Internet. If you do, please report your experience for the other users.