Making parts collide with a non-colliding part. Need help

Hi! I’ve been having some problems with Collision Filtering and am wondering if anyone can offer some solutions.

My problem is that I have a part which is set to CanCollide == false, however I want objects in one folder to collide with it. Nothing else should collide with the part.

This seems easy to fix, just use Collision Filtering right? I tried that, but it turns out (at least to my knowledge) that Collision Filtering only works to make a part non collidable with other certain parts. I want to make a part collidable with other certain parts.

Hopefully that makes sense.

Now, what I could do is set the CanCollide == false part to CanCollide == true, and use Collision Filtering to make every other part in the game not collide with the it (besides the objects in the folder). While this theoretically works, I worry that having five thousand or more parts added into a physics group would cause lag.

So…does having a huge physics group cause lag? Or preferably, is there a way to reverse Collision Filtering’s way of doing things? ᵖˡˢ ʰᵉˡᵖ

Oh, and one note:
physicsService:CollisionGroupSetCollidable("group1","group2",true) sure seems like it’d fix my problem, being that “true” means make the groups collide. but it doesn’t work. it just… doesn’t. without any errors too. both true and false do the same things. ???

It won’t cause lag. I’m using this for my game as well, there’s really no other efficient method of modifying object collisions aside from collision groups.

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Ok, thank you :]‎‎‎‎‎‎‎‎‎‎‎‎‎‎‎‎‎‎‎‎‎‎‎‎‎‎