End gateways appear to have major effects on framerate when there is a large group of them, unlike any other block I'm aware of, and it's not just from the dual beam animation.
How to reproduce
/fill ~4 ~10 ~4 ~-4 ~2 ~-4 minecraft:end_gateway
→ Causes a very significant frame drop.
- Now, try to run
/fill ~4 ~100 ~4 ~-4 ~2 ~-4 minecraft:end_gateway
→ It causes complete unplayability.