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If this is the case, why dont you just hop into Blender and then make a low-poly model there, and then export it to roblox?
When adding an object into blender, a little tab will appear, and there you can modify the amount of triangles / polygons on the model.
The segments and rings numbers are what you want to modify.
This is 32 segments:
And this is 12 segments:
You can then use the boolean modified in blender to mix all the parts together, which is pretty much a roblox union but better.
And, if possible, try reducing the amount of bones you spawn, either make them bigger or just have gaps.
Hope this helps 
Okay, thanks. I will try it. 
I have a ball that has 10 segments and 8 rings. Is that enough?
Really depends on how you want your game to look and perform.
Its up to you to find the right balance.
I’m a bit new to Blender, is there a way to group parts together in Blender?
Sorry for the late response here, but yes, there is a way to merge/group parts in blender.
This video displays how to use the boolean modifier to merge 2 parts together.
It also gets rid of any useless faces on the model too.
Okay great, that could help a lot.
Also… problem. The ball looks so… out of place. There’s a way to make it look smoother, but I don’t know how. Do you know how to do it?
Again, sorry for a late response.
You can make parts look smoother by right clicking then and then clicking the Shade Smooth option.
The low poly meshes idea worked. I can turn it into one mesh. But, I haven’t finished remaking the model yet… so not sure if it will lag or not.