Afaik, the collision has nothing to do with the GPU (unless you we’re talking special Niagara particles). It’s all CPU busywork.
will the box collision in the static mesh be faster than the capsule because of the triangles count? Or is the capsule collision not based on triangles?
Don’t quote me on that but there exist, apparently, special optimisations for primitive colliders like boxes & spheres, making them efficient. I’d assume that a capsule is 2 spheres and a box, potentially further optimised with some close-to-the-metal binary magic. I am guesstimating.
But yes, as a general rule of thumb using plain boxes can be a major optimisation providing the scope is grand enough.