Golang profiling - top10 shows only one line with 100%

I try to profiling my go library, to find out what is the cause of being so much slower than same thing in c++.

I have simple benchmark

func BenchmarkFile(t *testing.B) {
    tmpFile, err := ioutil.TempFile("", TMP_FILE_PREFIX)

    fw, err := NewFile(tmpFile.Name())
    text := []byte("testing")
    for i := 0; i < b.N; i++ {
        _, err = fw.Write(text)
    }
    fw.Close()
}

NewFile return my custom Writer which encodes data to our binary representation, even compress them, and write to file system.

Running go test -bench . -memprofile mem.out -cpuprofile cpu.out I get

PASS
BenchmarkFile-16    2000000000           0.20 ns/op
ok      .../writer/iowriter 9.074s

Than analysing it

# go tool pprof cpu.out 
Entering interactive mode (type "help" for commands)
(pprof) top10
930ms of 930ms total (  100%)
      flat  flat%   sum%        cum   cum%
     930ms   100%   100%      930ms   100%  
(pprof) 

I even try to write example.go app which is using my writer, and add pprof.StartCPUProfile(f) as is shown in http://blog.golang.org/profiling-go-programs but with same result.

What am I doing wrong, and how can I determine what is bottleneck of my lib? Thank you in advance