Changing the two commented lines in the code to the version used in this scripts makes this run twice as fast and makes it reach equal to C speed.

equal_to_c_speed_test.swift

// Test motivated by the following discussion: https://twitter.com/vojtamolda/status/1258516339402006528
// Compile script: swiftc -O equal_to_c_speed_test.swift

import Foundation

// var result = ContiguousArray<Int>(repeating: 0, count: 3001)
var result = [Int](repeating: 0, count: 3001)
for it in 0..<15 {
    let start = Date()
    result.withUnsafeMutableBufferPointer { buffer in
        for i in 0..<3000 {
    	    buffer[i] = it}}
    // let sum = result.reduce(0, +)
    let sum = result.reduce(0, &+)
    let end = Date()
    print(start.timeIntervalSince(end), sum)}