I've got my Mastodon and Lemmy servers hooked up to a postgres server that's running separately, and the CPU usage between the two is now about the same.
That actually proves that Lemmy is more efficient, because it's handling a hell of a lot more data. I don't follow a lot of big accounts, and I'm the only user on my server, so I rarely get more than 40 posts + metadata through Mastodon. On Lemmy, however, each post can easily produce hundreds of events to process because of comments and likes all federating out.
In total, Lemmy seems to be heavier, but only because it does more. Mastodon is super inefficient, especially with things like RAM. I think it has something to do with the framework and language it's running on; Gitlab seems to be using the same runtime and that eats through RAM like crazy as well.