Pretty easy question.
Which of these 3 items does load fastest, for example when 100 of them are placed?
Meshes, Union’s 
or Part’s ?
Poll is only for people who don’t know, and want to guess. People who do know, please let us know in the comments 
0 voters
Parts obviously load the fastest since they’re very basic and built into the game files. Meshs and Unions will have to download the mesh and load it into the game.
Parts load the fastest because they’re prebuilt into the Roblox client.
Now for MeshPart versus UnionOperation, it is a case by case situation. UnionOperations are probably faster if it’s a very basic shape. However if you are creating complex shapes, I suggest just using MeshParts and doing it in Blender as it’ll be more optimized and you also get the benefits of mesh instancing (loading the MeshId once and then reusing it for all the MeshParts).
Would making something in studio, exporting it as a OBJ back into studio to a mesh, work?
That works if you’re going to reuse the mesh. For example you create a UnionOperation of a bush and want to reuse it, then you should export as .obj and use it as a MeshPart instead.
However Roblox’s union tools aren’t the most precise and you’ll most likely get a more optimized version by using Blender to create the object.