I honestly didn’t see a post about this problem or I would have added it to the main post myself in a reply.
Simple fix for the lag issues with the elemental invasion event, add more of them.
The problem isn’t the number of players, the problem is you don’t have enough of these events happening at once, Primal Elements Events that is.
If you added more in major areas in the old world, it would spread out the number of players doing them so your servers wouldn’t be bogged down with lag!