But it can be noticed on mobile devices. By not caring about this, you are sacrificing performance.
Somehow I knew you’d be here.
To prevent this from being spam, anyone else reading this, it’s just better to learn blender and model something amazing in 5 minutes with less lag, rather than build the same thing in roblox and take 5 hours, have it lag your game, or possibly corrupt on you. (Corrupted unions is what finally pushed me to move to blender. That stuff annoyed the hell out of me.) Plus, blender has far more features like custom texturing. Blender also has much more applications in the wider development world.
In the scripting side of things, I’m not too educated, but other people have said to just use parts, so do what they say.
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Are you telling me, that if I make 2 or 3 parts into one union, on a baseplate, with nothing else on it, mobile will lag?
If that’s what you’re saying, have my doubts.
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No, I’m saying that mobile will be less performant.
I see what you’re saying, from a large-ish scale workflow perspective that makes sense. But I think the scale of what he is trying to do is relevant. If this is a one-off thing or a few unions total, I think your benefit would be small and time cost in learning blender would be significant. (but useful in the big picture)
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Perhaps, but with a few unions in question, the performance reduction may not be measurable or significant.
The OP was asking if unions have a performance impact, not if it’s noticeable in small quantities.
Well, if its not significant or hardly measurable then I wouldn’t say it will ‘slow down your game’
It makes a difference if your building something like this:
or something like this:
So I suspect the answer to OP’s question is, ‘it depends’
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Here is my answer to this. Union or Model, Which one is the best - #3 by RichardPRoosevelt
Unions create un-needed poly for no reason.
I’m not trying to say unions are better or in any way comparable to a blender creation, but …
in your 2nd video, your ‘comparable’ 600 face blender spheres look more like stop signs so you’d probably have more faces if you made them more smooth like the union that you imported. I’m sure it would still be lower but you need more than 600 it seems. And what the union did to the ‘box’, yeah, what is that? 
I think this is a good post with more details on the pro’s and cons to using unions and/or meshes:
What’s your use case?
Are you making something like, for example, multiple objects floating up and down all in sync?
In this case, you should group the multiple objects, set a primary part, and apply a script to the model to control the primary part of the model, which will move the whole model with it as long as it is anchored.
That’s the right way to do it and you should do it like that for ease of modifying/moving these objects in the future
You shouldn’t union them for this use case, but even if you do, there won’t be a measurable performance loss.
that’s it.
As for what other people here have said, unions aren’t bad and anyone who believes unions shouldn’t be used is flat out wrong.
Unions allow developers to easily and quickly create intricate models that would otherwise be either impossible or would take a long time to make.
For example;
Say we want some intention on a spaceship for a turret to be in, or a handle for a desk drawer or something similar, with unions it would be done somewhat like this
https://gyazo.com/6a93403886a543e0d1fd9e7e199f7508
How exactly should this be made in blender by hand without using the boolean modifier?
Unions have a use, are useful, and are often used in production games because they are simpler and easier to use when you understand them and use them correctly.
Incorrect use of unions is what creates a performance decrease in games.
You shouldn’t make a map entirely out of unions. A lot of beginner builders tend to see their maps have some lag, and they decide to union all the parts they can together to reduce lag. This, of course, is not a correct use case.
Instead of telling new developers to never use unions (like what @iGottic has done with his videos), and countless others telling people to stop using unions, maybe try to educate them on how to correctly use unions.
I’ve seen some people give actually good advice on the use of unions, like only using unions on high part count models which will be repeatedly used on a map, like lamp posts, trash cans, stuff like that. This can really help new developers that don’t know how exactly unions work and decide to never use them because of bad advice.
@iGottic second video compares two different meshes entierly?
His recreation of the object consists of 8 spheres of 6 rings and 10(?) segments and a cube and he compares that to a unioned object of 8 roblox spheres made of 12 rings and 24 segments and a cube.
Either this is pure ignorance, done intentionally to make the gap between face counts as high as possible, or was an actual mistake, and if so, I hope this is addressed.
Better use of a video like this would be to inform people that simple models shouldn’t be unioned, unions should be used for models with a lot of parts, cylinders, or spheres that you will use repeatedly across the map. Right now this video only shows the byproduct of incorrect use of unions
In conclusion, Use meshes when possible, but don’t be afraid to use unions. Be smart when using unions, use them only when actually necessary. You’ll find they play almost as important of a role as meshes do.
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This statement is incorrect.
When you create a union, it goes through a constructive-solid geometry (CSG) algorithm. This is a binary tree which determines booleans (holes) and unions (stuff put together).
Unfortunately, this is prone to a few issues, such as:
- Multiple vertices in a single vertex location (for example, I notice often 3-5 vertices per corner)
- Unneeded triangles to construct a polygon (due to Roblox’s poor triangulation methods)
- Collision issues
These issues contribute masses of lag.
That was a marginal difference. In the video, notice this:
The box has way too many triangles – in reality it should only have 12 (2 per side, 6 sides).
I have had maps produced for me that I have simply denied due to usage of unions. Unions are always laggy and I have recorded this in practice.
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Yes creating Unions will slow down your game as the parts you used to make them are still saved. You should only create unions in moderation and if you’d like to somewhat improve the union just follow these steps.
1: Right click on the union on the explorer tab and select Export Selection
2: Add a MeshPart into the explorer
3: Click on the MeshId property and find the file you exported
This will convert the Union into a mesh and should halved the file size.
The performance goes as follows. BasePart > MeshPart > Unions
However if the Union you created is very complex which has a lot of triangles then you may need to export it to Blender to remove the unnecessary faces. The steps to do this is a lot trickier to explain so you may need to look it up.
This uses the same CSG system (I’d assume) and is just as bad as using unions.
I personally use unions scarcely and only when necessary, but unions on their own are not bad both (yours and dev) points are valid.
If you obsessively create hi-tri unions (especially using parts that have hi-tri counts, like spheres) then you’ve more or less started off on the wrong foot already.
You shouldn’t have hi-tri unions in general, unioning many parts together, regardless if you intend on re-using said unions, probably will not work out well.
Unioning and keeping a low-tri count and re-using said unions is ideal, especially in instances where you need to union, either because you don’t have blender or otherwise.
Roblox’s system to union stuff is still really poor that’s for certain but it’s not bad if you use it for specific circumstances.
The box is a good example where CSG fails currently, where there are either many parts and/or circles/spheres being unioned together. But that doesn’t apply to all instances of using unions.
this doesn’t change anything performance-wise.
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Yes, unions will actually slow down your game and even laggier than using parts!
Sorry I haven’t replied to anyone, my internet was out 
, thanks for all the responses guys!
Unions are not a great choice, since they create triangles/vertices where unneeded, and they actively calculate geometry.
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