What's the recommended way to use s3manager.Downloader?

Question

Recently, I tried to improve upload and download experience, and I found that s3manager.Uploader is so amazing to improve the upload experience for larger objects by parallelizing them. The below code works well. We can make full use of our bandwidth (far better than PutObject).

uploader := s3manager.NewUploaderWithClient(s.Client, func(u *s3manager.Uploader) {
    u.Concurrency = 128
})

So I tried to use the similar way to improve the download speed, but it seems that whatever concurrency is used it can't accelarate the download speed. The download speed is always below 15 MB/s.

By the way, GetObject API works pretty good with very large objects, it makes full use of our bandwidth when we download large objects, GetObject can reach bandwidth limit about 1250 MB/s.

So I tried to do some benchmark test, and the above image is what go tool pprof shows.

func (b *WriteAtBuffer) WriteAt(p []byte, pos int64) (n int, err error) {
    pLen := len(p)
    expLen := pos + int64(pLen)
    b.m.Lock()
    defer b.m.Unlock()
    if int64(len(b.buf)) < expLen {
        if int64(cap(b.buf)) < expLen {
            if b.GrowthCoeff < 1 {
                b.GrowthCoeff = 1
            }
            newBuf := make([]byte, expLen, int64(b.GrowthCoeff*float64(expLen)))
            copy(newBuf, b.buf)
            b.buf = newBuf
        }
        b.buf = b.buf[:expLen]
    }
    copy(b.buf[pos:], p)
    return pLen, nil
}

Here's my question, Why is s3manager.Downloader so slow, is data race slowing down the WriteAt interface? What's the recommended way to use s3manager.Downloader?