Can you white list the .touched event?

I have a game with many slow moving projectiles. There are also many objects that are in the line of sight of these projectiles that they intersect with tripping the event that are ignored. I am wondering if I can set a few objects that will register the .touched event. I don't believe .magnitude will solve my issue as even 50 projectiles, with 50 walls on screen is still 50*50 = 2500 tests at the slowest 5 times per second is 12500 computationally intense tests a second.