Slow website and thumbnails not displaying

The thumbnails to my latest posts are no longer showing for a some reason. I am not very good with coding, but I installed a a plugin to improve my website speed, and that seems to be when the problems started. I removed the the plugin, but still the thumbnails are not showing for new posts.
I also hope you can help with the slow pace at which the site is loading.

The page I need help with: [log in to see the link]