I use addthis.com on sites and I make sure to put the javascript near the bottom of the page since it's not too important to have that file loading earlier. Try moving your js to the footer. Considering how many websites use these share buttons I don't think they should be affecting your website that severely. In Google Chrome developer tool under Network if you reload your page you can see what exactly is causing the slow page load.
I see the problem is your sharethis code is at the top of each post and it's displaying the snippet multiple times on your home page for each post. That means your site is making a lot of connections out bound when a visitor connects. Put sharethis code into the bottom of your posts at the end. And display an excerpt of the posts on your home page. You can customize the length of the post to show and more with WordPress's built in functions.