No they actually cause lag because of there idle animations. All the larger dinos do but the bronto, titano and diplo are the worst next to maybe a squid.
It doesn't really affect anything if you have like 10 on a server but if its like official where tribes will have 200 broncos it will completely lag out there server just like 2000 turrets firing at once.
Structures cause lag.. flying 2 fast across the map causes lag rocket launcher cause lag.. to many tames cause lag.. What doesn't cause lag in this game doesn't matter what server your on.. high populations cause lag
It all causes lag every little thing you do in the game add to how much you're xbox has to process aka you max it out it lags. Bad Internet causes lag too.
Brontos, Rock Elementals, and Doedics all cause lag with high harvesting settings. If your server is mildly to not boosted it won’t matter. If a couple of seconds is your definition of “lag” well play a different game.
Luckily there are 7,000+ servers to chose from. It doesn't matter who is right or who is wrong. There's a ton of choices out there. Pick the one you like the best.