Put the summary of the bug you're having here
What I expected to happen was...:
Describe what you thought should happen here
What actually happened was...:
Describe what happened here
Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ... Summon Alot Of Iron Golems
3. ... Summon Too Much Enities
"Summoning a lot of entities" is an expected cause for lag. More entities in general means more information for your computer to update and render, and eliminating this lag is not really feasible.
If you could provide details regarding a more specific performance issue (like a specific entity, in comparison to other entities, causes more lag), please create a new report, and we'll be happy to review it. 🙂