Federation takes almost no server load. It’d all the people loading images and comment threads that cause the server to slow down. Yes federation can break, especially due to unexpected code changes, but typically using a federated, lower user count site helps over all.
0.19.1 has a problem that causes higher than expected server load, as illustrated in my first post showing their utilization graphs.
As for a few months ago, “someone” was DDoSing lemmy.world almost constantly but i guess they ran out of money or got bored.