When you build a result using a for loop, the first item might need a specific treatment.
The most efficient solution is to use an if statement to avoid code duplication.
Whatever the test, no significant difference appears.
The unfair solution doesn't take the full hypothesis into account and gets a 10% improvement.
src>go test -bench=. ./firstitem
Results
go version go1.12 windows/amd64
BenchmarkFirstItemBefore-4 20000 86551 ns/op
BenchmarkFirstItemIfOnIndex-4 20000 88000 ns/op
BenchmarkFirstItemIfOnArray-4 20000 88107 ns/op
BenchmarkFirstItemBeforeUnfair-4 20000 70460 ns/op
PASS