my testing down to a couple of brief XCTests Seems XCTest is not a good tool to measure micro performance.
With this simple code:
import Foundation
import simd
class MyClass {
		func measure1(code: ()->Void) -> Double {
				let start = clock_gettime_nsec_np(CLOCK_UPTIME_RAW)
code()
				let end = clock_gettime_nsec_np(CLOCK_UPTIME_RAW)
				let time = Double(end - start) / 1_000_000_000
				return time
		}
		func measure(code: ()->Void) {
				var times: [Double] = Array(repeating: 0, count: 10)
				for i in times.indices {
						let t = measure1(code: code)
						times[i] = t
				}
				let avg = times.reduce(0, +)/Double(times.count)
print("average: \(String(format: "%.6f", avg)), values: \(times)")
		}
		func testPerformance_double() {
				//...Exactly the same as shown
		}
		
		func testPerformance_simd() {
				//...Exactly the same as shown
		}
		
}
let myObj = MyClass()
print("testPerformance_double:")
myObj.testPerformance_double()
print("testPerformance_simd:")
myObj.testPerformance_simd()
The output:
(Tested on Mac mini (2018), Xcode 12.3, macOS Command Line Tool project, Release settings.)
testPerformance_double:
xL: 0.6408629334044746, xR: 0.735714601868435, increment: 0.05068415313623026
average: 0.000156, values: [0.00023855, 0.000140175, 0.000211979, 0.000193554, 0.000178274, 0.000118627, 0.000157364, 0.000136575, 9.3264e-05, 9.3267e-05]
last result: 35599.67430242713
testPerformance_simd:
vL.x: 0.2631681036829726, vL.y: 0.026889765245537545, increment: 0.04897156513471709
average: 0.000156, values: [0.000269612, 0.000122409, 0.000192792, 0.000141103, 0.000124708, 0.00012638, 0.000124496, 0.000211614, 0.000122489, 0.000122688]
last result: SIMD2<Double>(15474.766375371206, 40414.973240285304)
(I have chosen one interesting result from a few tries, but no significant differences in all other tries.)
I'm afraid your code is too simple to measure micro performance, Swift compiler would reduce many parts of code inside iteration.
Performance improvement is a very interesting topic, but you may need some better ways to explore.
