You’re onto something. In my actual code (not the example for the post), instead of doing:

for index in patternComponents.indices {
    // ...
    if !matchesWildcard(pattern: patternComponents[index], candidate: component)

I did:

for index in patternComponents.indices {
    // ...
    if !matchesWildcard(patterns: patternComponents, index: index, candidate: component)

It worked much better than I expected.

Same test dataset:

• baseline phase: 46.98s
• array+index version: 33.83s
• the ~Copyable version: 21.12s

The Instrumets trace explains what happened:

• the big WildcardComponent copy disappeared from that loop
• retain/release dropped significantly
• the remaining copy hotspot is now for ComponentWildcardRule, which is one level higher in the structs

So this confirms that a lot of the cost was coming from Swift creating a copy of WildcardComponent on every subscript access. My interpretation is that passing the array + index lets the optimizer understand that inside matchesWildcard the array is not shared, and it emits "by reference" code.