If the website pages are created in a certain pattern you can simplify the process of sitemap generation.
for example :
http://www.example.com/?id=1,
http://www.example.com/?id=2,
http://www.example.com/?id=3,
....,
http://www.example.com/?id=5000000

Just input the minimum and maximum values of the numbers. In this case the process of sitemap generation will take a few seconds and the pattern will look like this: http://www.example.com/?id={0}

In other case, when the website URLs include words (say article titles, topics) you can simplify the process of sitemap generation having only the list of those words.
for example :
http://www.example.com/article/title_1,
http://www.example.com/article/title_2,
http://www.example.com/article/title_3,
...,
http://www.example.com/article/title_5000000

In this case the pattern will look like http://www.example.com/article/{0}