Mastodon due to the decentralized nature can result in a significant extra load on your site if someone posts a link to it. Every Mastodon instance where the post is seen (which can be 1 but also 100 or 1000 or …) will request not only the page and sub-sequentially the oEmbed json object to be able to show a preview in Mastodon. The page requests should not an issue as you surely have page caching, but the oEmbed object lives behind /wp-json/ and as such is not cached by page caches. The solution; the WP Rest Cache plugin and one small code snippet (for now). A lot more info can be found in Donncha’s excellent post on the subject.