Gravatar is the default way of handling comment avatars in WordPress. A very cool feature in itself, but it can have an impact on your site speed if your posts receive a lot of comments.
This is simply a result of your site having to display tens of avatar images. You can reduce this load by disabling those Gravatar images completely.
Handle this is your wp-admin: Go to Settings / Discussion, and deselect the “Show Avatars” box.