如何优化图片替换性能，避免大量图片处理时的速度下降？

var client *oss.client
var once sync.once

func init() {
  once.do(func() {
    var err error
    client, err = oss.new(endpoint, accesskeyid, accesskeysecret)
    if err != nil {
      fmt.println("error:", err)
      os.exit(-1)
    }
  })
}

func upload(url string) string {
  obj := fmt.sprintf("article/%d%d.png", t1, long)
  channel := make(chan string)
  go func() {
    bucket.putobject(obj, bytes.newreader(readimgdata(url)))
    channel <- "https://" + bucketname + "." + endpoint + "/" + obj
  }()
  return <-channel
}

type Cache struct {
  sync.Mutex
  data map[string]string
}

var cache = Cache{data: make(map[string]string)}

func Upload(url string) string {
  cache.Lock()
  img, ok := cache.data[url]
  cache.Unlock()
  if ok {
    return img
  }
  obj := fmt.Sprintf("article/%d%d.png", t1, long)
  channel := make(chan string)
  go func() {
    bucket.PutObject(obj, bytes.NewReader(ReadImgData(url)))
    cache.Lock()
    cache.data[url] = "https://" + BucketName + "." + Endpoint + "/" + obj
    cache.Unlock()
    channel <- "https://" + BucketName + "." + Endpoint + "/" + obj
  }()
  return <-channel
}