如何在golang中将大文件传输到http客户端

I have a handler function which looks like below:

func DownloadFileHandler(w http.ResponseWriter, r *http.Request) {
    SetResponseHeaders(w, r)
    if r.Method == "OPTIONS" {
        return
    }
    var err error
    var responseCode int = http.StatusBadRequest
    vars := mux.Vars(r)
    file := vars["file"]
    var n int64 = 0
    var reader io.Reader
    Filename := "images/" + file
    f, err := os.Open(Filename)
    if err != nil {
        err = errors.New("File not found")
        goto Error
    }
    reader = bufio.NewReader(f)
    settings.WriteDebugLog(time.Now())
    n, err = io.Copy(w, reader) // this takes a long time if the file is large
    settings.WriteDebugLog(time.Now())
    settings.WriteDebugLog(n / 1024 / 1024)
    responseCode = http.StatusOK
    settings.WriteLog(r, responseCode, nil, GetCurrentFuncName())
    return
Error:
    WriteErrorResponse(w, responseCode, err)
    settings.WriteLog(r, responseCode, err, GetCurrentFuncName())
}

But if it is a large file (more than 1GB), the io.Copy takes quite a long time (best case: 1G 13 seconds shown below) and the download process in browser doesn't show up until the process of io.Copy completes. Is there a way to achieve my goal more efficiently? I want the download process to show up in my browser right after I send the download request.

2017/05/16 22:32:29 Debug: 2017-05-16 22:32:29.967499719 +0900 JST
2017/05/16 22:32:36 Debug: 2017-05-16 22:32:36.980914163 +0900 JST
2017/05/16 22:32:36 Debug: 1097

I tried calling io.Copy with "go io.Copy" then the download process shows up soon but the file size is 0.