Per poly collision is complex collision. Unreal has 2 collision types, simple and complex. Complex collision, when active, calculates collision trace against the mesh itself. So if it’s a 400,000 triangle mesh, and a ball hits it, that ball’s collision is calculated against 400,000 triangles every time it hits. Which is very unperformant.
That’s why we have simple collision, which is another, simpler shape on top of the actual mesh, in this case, a box, which serves as the collision for the mesh. Now the ball only has to collide with a 12 triangle mesh instead of 400,000. Which is much more performant. You can mark my above post as the answer so that others can find it if they come across the same issue