Part Count or Unions? Which produces more lag?

We are currently building a near-replica of the City Melbourne in Australia, which has some amazing architecture resulting in some of our builds to have alot of parts. I’m enquiring if Part Count or Unions cause more lag and overall some suggestions in decreasing lag for lower end PC’s. We have some buildings converted into mesh parts, but this is a very tedious and time consuming task.

Any tips or suggestions?

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Based on other things I read, unions and especially meshes could lower performance because the game has to load the files for those meshes.

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So having more parts would be better in some cases?

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Having the parts on there own will always produce more lag then unions. Unions put everything together so produces less lag. Also more higher models (for example from blender) will produce lots of lag but lower polley ones will produce less.

I would not Union random parts to increase performance. Yes, it decreases part count but it decreases performance. If it’s a complex model, consider using blender.

Csg v2 is not good at creating performant parts so I would not Union random parts. Parts are made to be performant already.

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Agreeing on what @grif_0 making unions will change some things like the color as well and stuff like that.

Would creating a building in CSG then exporting alot of parts and re-importing them help with performance?

  1. No number of any kind of object can accurately tell you what the performance cost is because there’s too many variables on how to use them. Use Memory Use stats to determine what’s best for your game.

  1. To clear misconceptions:

This can be both true and untrue, due to variables in how they’re used and instanced, and why we all need to avoid blanket statements. Sometimes MeshParts are more efficient, sometimes Parts are. It depends on how you use them.

This is usually an inaccurate way to describe how it works. If you’re using a lot of Parts in an asset that’s repeated often, it’s sometimes better to use a MeshPart. Unions are just inefficient MeshParts. However, do not use Unions simply because you think less objects = better memory use. That’s not how any of this works.


I’ve found that for complex assets made with a large amount of Parts can benefit with having the more complex pieces be turned to MeshParts instead. That’s really dependent on how it’s applied though.


  1. Memory Use and game performance is a complex topic, thankfully, I did a whole RDC talk on this very thing. Watch it here:
    https://developer.roblox.com/en-us/videos/building-worlds-for-game-performance
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From my experience of building massive cities, with good detail and no lag, unions are the cause of lag.

My cities do not have any unions what so ever, but as soon as I put a union in, even if its just 1, the performance goes down.

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Do you mainly use parts? Parts converted into meshes?

Also that looks awesome!

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Thank you.

I use parts, and I use meshes. No unions at all.

I’ll try not using unions at all, as i can agree with you on the fact they impact performance from testing of my own.

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