After more investigation, it seems that the island alone is causing the issue. I’ll keep removing objects on it, beginning with scripts, meshes and unions, to determine the exact cause.
Edit: I already know it’s not caused by any script, mesh or union. I also don’t believe anymore it could be caused by the size of the place, since the majority of the objects was removed.
Edit2: I finally know the cause! It’s caused by some very large triangles I used for the island (they are 1000 studs+ long and wide). I am sure now that it is NOT caused by the size of the game, only because there is apparently a serious issue with Roblox when it loads very large parts.