Need an efficient way of handling a projectile passing through a large number of objects

This is in 2D.

- To keep this simple, let's say there are two classes: unit and projectile. Units have a circle collider, projectiles also have a circle collider but it's small enough that we can just say they're a point.\
- Units and projectiles have an 'owner' property that corresponds to a player. There are potentially many players and their alliances are flexible, but for the purpose of this post lets say there are two players: red and blue.
- A projectile should have no effect on units belonging to its owner, but when it hits a unit belonging to the enemy, it is destroyed and causes damage (etc). So a red projectile will pass through red units and hit blue units and vice versa. Projectiles do not interact with each other (physics collision turned off between layers).
- Projectiles have trigger colliders and units have regular colliders, so I'm using OnTriggerEnter2D to trigger the interactions. All that happens in OnTriggerEnter2D (on the projectile) is: check if other collider is attached to a unit (CompareTag), check owner (compare ints), if it's not the same owner deal damage etc.

The problem I'm having is that when many red units are bunched up and fire at a blue unit, the game lags into oblivion. According to the profiler, the problem is in Physics2D.FindNewContacts; i.e. there are too many trigger collider interactions, as far as I understand. There are hundreds of projectiles and units, and most of the projectiles pass through several friendly units on their way to their target, so it does kind of make sense. My question is: what is an efficient way to handle this situation?

Right now I have disabled physics interaction between units and projectiles and am manually range checking every projectile-unit pair (naive approach). It is significantly faster but still slow (obviously). I'd prefer to use the physics engine, but I need to find a way to get it to perform properly. It's a fairly simple scenario, and I feel like it should be quite common. Am I missing something obvious? I've been chatting with some people on the gamedev discord and tried changing some physics settings but nothing has really yielded any significant improvements.

The strange thing is that what I'm making is a recreation of something I already developed in Processing (Java based programming environment for visual art), and in that I'm doing a similar naive approach scanning through every unit on every projectile every frame and measuring distance, and with far more units than I've tried so far in Unity and that runs orders of magnitude faster.

Thanks in advance

 

Thanks for the reply. Yeah I am using the layer collision matrix - projectiles don't interact with other projectiles.
And yes, you're right - it gets up to like 20-50k contacts.

So you're saying don't use the physics system to control projectiles? (make them kinematic?) I suppose a circle cast would work. If I circle cast from a projectile that's overlapping/completely contained within a unit, will it return that unit?