Yes, depending on the unions, it can slow down the game, especially if you have a ton of them.
The hitboxes/collisions on them and rendering can pose some problems to performance. That doesn’t necessarily mean you should delete them all, you can use some of them but if its a complex shape, I’d just recommend making them in blender as a mesh instead of a union since it’ll be easier to adjust it.
You can “fix” some collision problems/rendering problems by making sure the transparency is between 0 or 1 which can render/not render it. You can change the render fidleity to “low/automatic” (forgot what name). You can also make the collision “box”.
Unions can only cause lag whenever it has many triangles set on the unions. About the same thing applies with meshes. In other words, the more complex it is the laggier it gets.
Collisions are something to take consideration of. If your mesh or union is very complex, there’s a chance that the complexity can cause a laggy collision box, due to its complex shape.
You can check the decomposition geometry by a single checkbox, to see all the collisions. If it’s a background object for viewing, do check CanCollide off or set collision to box.