Hello,
Sitemap Generator script is crawling your site (fetching the pages with http requests) to make sure that all pages are included in sitemap, so the main factor is how fast the page of your site is loaded (including
page generation time). Since the script is running on the same host where your website is running, the network connection is performed significantly faster than desktop application running on your computer.
You will find more details here:
http://www.xml-sitemaps.com/forum/index.php/topic,95.msg387.html#msg387Please note that you can
greatly improve generation time using "Do not parse URLs" (pages included into sitemap, but not downloaded by the script) and "Exclude URLs" (do not include into sitemap).