Due to the 5000 poly count limits when importing custom meshes into Roblox, my team is thinking about chopping up high poly count custom mesh into multiple pieces and bring them into Roblox. We are not too familiar with Roblox but there seem to be two ways to put these pieces together into one. Namely union and welding.
What are the major differences between the two? I’m particularly interested in the performance differences in both methods and how they affect the physics engine if at all.
You cannot union imported meshes together, so if you use this method you’re likely to end up welding MeshParts together.
Unions tend to create extraneous geometry, which can have an impact on performance. MeshParts will always be superior in that regard.
5000 polys is quite a lot though. Are you sure you can’t decrease that without losing detail? A lot of players will suffer if you have an excessive polycount.
Our modellers come from modelling high poly count games. We are trying to see if at least the weapons can be high poly count in the game as they look quite nice while probably keeping the environments low poly. Would this be too much of a burden on the physics engine still?
Actually on second thought, it would be the static buildings and stuff that are anchored that could be high poly count put together I guess? Since those things are like no brainers for the physics engine. Anything moving and animating would be a burden to the engine i’m guessing.
It’s more an issue of rendering. Weapons are relatively small, and buildings tend to be relatively boxy, and collision geometry on them can be simplified to the hull shape of the mesh, or even just a simple box, just by changing a single property.
Rendering them however is completely different. A lot of players on Roblox are using very low end hardware, so having a large, unnecessary number of polygons when you can achieve the same look and effect with far fewer is akin to shooting yourself in both kneecaps.
To put this in perspective, I tried using 5000 polys on a few foliage parts in a small test map and in only maybe 200 parts my outdated (but realistically so for Roblox’s demographic) GPU was struggling to keep 30 fps.
qqtt991 is correct, you really shouldn’t be aiming for 5000 triangles for a gun mesh - or any mesh. If you’re looking to up the detail, use textures instead.
Roblox isn’t Unreal Engine, and triple A graphics shouldn’t be an end goal on Roblox, as unfortunate as that is. A more realistic end goal is “iPhone 5 compatible” 
At least you don’t have to redo all of your high-poly meshes from scratch, if you (or your modelers) need help, DM me any time 