@swift-5.3.3 -O
import Foundation func measure(_ body: () -> Void) { let start = Date.timeIntervalSinceReferenceDate for _ in 0 ..< 10 { body() } let end = Date.timeIntervalSinceReferenceDate print((end - start) / 10) } func sumOfArray(_ a: [Int], indices: [Int]) -> Int { var sum = 0 for i in indices { sum &+= a[i] } return sum } func sumOfArraySlice(_ a: ArraySlice<Int>, indices: [Int]) -> Int { var sum = 0 for i in indices { sum &+= a[i] } return sum } let n: Int = 1_000_000 + (1 ... 100).randomElement()! let a: [Int] = .init(1 ... n) let b: ArraySlice<Int> = .init(a) let indices: [Int] = a.indices.shuffled() var sum = 0 measure { sum &+= sumOfArray(a, indices: indices) } measure { sum &+= sumOfArraySlice(b, indices: indices) } print(sum)