// main.swift
import Foundation

class ByteReader {

    let data: Data

    var offset: Int

    init(data: Data) {
        self.data = data
        self.offset = data.startIndex
    }

    func byte() -> UInt8 {
        precondition(self.offset < self.data.endIndex)
        defer { self.offset += 1 }
        return self.data[self.offset]
    }

}

let byteReader = ByteReader(data: Data(count: 10_485_760))

for _ in 0..<10_000_000 {
    _ = byteReader.byte()
}

$ swiftc --version
Apple Swift version 4.2.1 (swiftlang-1000.11.42 clang-1000.11.45.1)
Target: x86_64-apple-darwin17.7.0
$ swiftc -O main.swift
$ time ./main

real    0m0.948s
user    0m0.920s
sys 0m0.022s
$ xcrun -toolchain org.swift.41320180727a swiftc --version
Apple Swift version 4.1.3 (swift-4.1.3-RELEASE)
Target: x86_64-apple-darwin17.7.0
$ xcrun -toolchain org.swift.41320180727a swiftc -O main.swift
$ time ./main

real    0m0.450s
user    0m0.419s
sys 0m0.018s