Exactly what @RobloxKonnie said. Trust me, when I first used studio, I hadn’t realized the actual scope of Roblox’s capabilities when it comes to its engine, so you may have to reduce your part count or create a chunk system. Good luck : )
If you have a lot of parts, instead of using Unions (which don’t affect performance) instead use meshes. a good explanation and tutorial on converting parts to meshes can be found here: Studio Tips Guide #1 - Converting Models to Meshes though I also recommend enabling StreamingEnabled, it only loads BaseParts that are within a range of the player (i don’t exactly know how it works, but I do know it can help for games with bigger maps/more parts).
Edit: i just realized someone already suggested StreamingEnabled but ima keep it in this reply anyways.