I don’t know if it causes server wide lag. I imagine there’s a small hiccup of strain when they all simultaneously aggro though. Usually 10 pets get triggered per maelstrom spawn.
I honestly experience way more lag during a surge due to loot/bodies.
I cut out the pets that were somewhat useless, like spiders and elephants to reduce that server strain.
I do play on a server with streamers so I can test it out this weekend maybe. If it does cause lag, there’ll be evidence of it I can find.