如果语句使基准测试中的代码执行速度更快，是否从未触发？ 为什么？

I have recently started the Go track on exercism.io and had fun optimizing the "nth-prime" calculation. Actually I came across a funny fact I can't explain. Imagine the following code:

// Package prime provides ...
package prime

// Nth function checks for the prime number on position n
func Nth(n int) (int, bool) {
        if n <= 0 {
            return 0, false
        }

        if (n == 1) {
            return 2, true
        }

        currentNumber := 1
        primeCounter := 1

        for n > primeCounter {
            currentNumber+=2
            if isPrime(currentNumber) {
                primeCounter++
            }
        }
        return currentNumber, primeCounter==n
}

// isPrime function checks if a number 
// is a prime number
func isPrime(n int) bool {
    //useless because never triggered but makes it faster??
    if n < 2 {
        println("n < 2")
        return false
    }

    //useless because never triggered but makes it faster??
    if n%2 == 0 {
        println("n%2")
        return n==2
    }

    for i := 3; i*i <= n; i+=2 {
        if n%i == 0 {
            return false
        }
    }
    return true
}

In the private function isPrime I have two initial if-statements that are never triggered, because I only give in uneven numbers greater than 2. The benchmark returns following:

Running tool: /usr/bin/go test -benchmem -run=^$ -bench ^(BenchmarkNth)$

BenchmarkNth-8           100      18114825 ns/op           0 B/op          0 

If I remove the never triggered if-statements the benchmark goes slower:

Running tool: /usr/bin/go test -benchmem -run=^$ -bench ^(BenchmarkNth)$

BenchmarkNth-8            50      21880749 ns/op           0 B/op          0

I have run the benchmark multiple times changing the code back and forth always getting more or less the same numbers and I can't think of a reason why these two if-statements should make the execution faster. Yes it is micro-optimization, but I want to know: Why?

Here is the whole exercise from exercism with test-cases: nth-prime

Go version i am using is 1.12.1 linux/amd64 on a manjaro i3 linux