as said by Diesoft:
All right so here is the thing, when you union two parts to make one union you create an object which has a unique union data. I guess you can see it as a blueprint for how the union should look and it's physics as well as size and so on. Now imagine that you copy this union, this creates two identical unions, for as long as you don't make any changes to this union (colour, transparency, reflectance or material/texture not included) the union data will remain the same and when you load the place ROBLOX will only fetch the union data from the ROBLOX database once, then duplicating the data. This way ROBLOX don't have to fetch the same data over and over and this results in your game loading way faster.
Moral of the story: Low amount of unique unions = good
Now regarding the performance issue, a union is still a bunch of polygons, you can't get around it. And it is the polygons that has to be rendered in order for you to actually see shapes. It is the rendering part that cause lag, and ROBLOX render ONLY the objects you can see. This means that if you for example only have half of the object in view, it will render the remaining part of the object. Now imagine if you union a lot of parts together that has a massive size combined. This means that as long as you just have a corner of the union visible, it will render the entire thing.
And since ROBLOX has come so far since the Featherweight update (back when bevels was removed, if any of you remember that) it doesn't matter if you have thousands of primitives. The place won't lag as badly as it used to so don't worry about the primitive count too much (obviously you will have to at a later time but if you're under 10-15k you're fine).
Now when should you union things?!?!?!
Only when it is needed, for example moving objects like guns or vehicles like cars or planes. The less amount of moving primitives = the less lag = very good = what you want. You should also resort to using CSG if it means that you're going to create a shape that require CSG (obviously). Also if you are making a holo for example! The maps, you want to keep the maps that will be loaded in at a low primitive count so unioning these could be a good choice. However be wary of physics errors, invisible walls may appear so always check for invisible walls. But if you do this it means that less primitives will be loaded in = better performance.
SO! CSG 101, and there is more to it but this is all you need to know.
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