It’s entirely dependent on what’s being made. Parts benefit from their simplicity and ease of use, but lack the ability to have their topology edited. When a player is rendering your game, all faces are rendered as triangles. When making complex objects outside of Roblox Studio, modelers are able to optimize their model by deciding exactly where these triangles go. The thing with parts is that you are not able to edit how each face is constructed. Each face of a basic Roblox part will always have two triangles, meaning if you have a lot of parts, you can end up with a ton of unnecessary triangles. This also means that you cannot remove faces that the player does not need to see. Now, reconstructing your assets in Blender does not magically make your game more optimized; there would only truly be a performance difference if your mesh has fewer triangles than your original asset. Hope this helps!