I have encountered a bug with unioning am I am looking to know if there are any fixs, besides just “Unioning fewer objects”
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What are your pc specs? Cause it could be your computer.
That might be because the parts are overlapping, try offset one of them by 0.001 in the z axis
There is no way around making the specific unions less buggy, to my knowledge.
That being said, it looks like the problem you’ve got there is that 2 different parts with different textures are being made to share the same co-ordinate on one axis, making them clash, to solve that, simply move one slightly further or back.
Generall with unions and lags, only alternative you have is to make other aspects of your game less laggy to my knowledge.
One way of reducing lag would be to reduce the render distance or create fog (I don’t know about render, but for fog, look in the properties of “lighting” in the explorer tab). This means that the game will have less trouble rendering. Of course, there’s also free models which are supposed to lag the game out too.
I have no expertise in meshing whatsoever, but I’ve heard it is possible to convert unions into meshes which are then less buggy, though again, I have no idea how to do so.
Maybe you might have unioned allot of models which ROBLOX does not like which causes it too go a little awkward, maybe try to union it a bit simpler.
I don’t know if this may help the most but I have tried what to say.
This isnt Z fighting. It’s just a bug probably with his PC / Studio
Same thing happened to me, I uninstalled ROBLOX Studio from my laptop and then reinstalled it. It worked just fine and has no problems - hopefully I can keep it this way! 
I’m not sure, it could be Z-fighting, I’ve never come across something like that without it being a Z-fighting problem, but I suppose the only way to find out is to experiment, and again, that’s why I included other ways of dealing with lag in my response!