There isn’t an instant magical solution that works for everyone, I suggest showing what your game looks like.
For starters;
Turn off CanCollide and CanTouch (AND CANQUERY!) for instances (parts/meshparts/unions) that don’t need collisions. (ex: tree leaves, bushes, clouds) (if it’s a MeshPart, then turn CollisionFidelity to Box)
Turn off CastShadow for instances that don’t need shadows. (super small parts, parts in complete darkness, etc)
If you have MeshParts that aren’t important to the game, turn their RenderFidelity to Performance. Do not use Precise RenderFidelity unless it’s a big object that’s visible in every direction and at all times. You should be using Automatic over Precise in 99% of cases.
I don’t really want to rewrite an essay, so if your game has a bunch of MeshParts and you’re looking to make them as performant as possible, please check out my tutorial topic here.