sitemap generator crawls your site similar to normal visitors, i.e. opening every page of the site to find nested links. So, working time is mostly related to the time it takes to open a page on your site. i.e. if your pages (without images/css/etc) are opened in average of 1 second, crawler will get 3,600 pages in hour. Correspondingly, if your site serves pages faster it can crawl more pages. Please check how long (approximately) it takes to load pages on your site, may be it just started working slower.