This is not a stumbleupon issue. log out if you are using the stumbleupon toolbar and proceed as follows.
1. Your blog is taking a long time to load because you have far too many posts on the front page. You can reduce the number onof posts on the fornt page by changing the number here on this page > Settings > Reading
2. You can also spilt the content in your posts by inserting "the more tag" into your posts. That will result in beginnings of posts on the front page of the blog with a "read more" link that readers can click to read the whole post on it's own page.
Google let us know that pageloading time became a pageranking factor last year. Here's a post that lays our other strategies for reducing it. http://onecoolsitebloggingtips.com/2009/06/13/blogging-strategies-for-reducing-page-loading-time/