How to limit download speed using UnityWebRequest? - unity3d

Question

Most files of our game is download during playing. If the download speed has no limit, there is not network traffic space left for the game play protocols... I tried to use the "DownloadHandlerScript", and provide a array of "preallocatedBuffer" to control the accept content size from web per frame, but it look like unuseful...Can anyone help me please?
Here is the DownloadHandlerScript I use:
class CustomDownloadHandler : DownloadHandlerScript
{
private byte[] _cacheBytes;
private int _index;
public CustomDownloadHandler(byte[] preallocatedBuffer) : base(preallocatedBuffer)
{
}
protected override void ReceiveContentLength(int contentLength)
{
_cacheBytes = new byte[contentLength];
}
// this method will be called once per frame to hand chunks of that data to script.
protected override bool ReceiveData(byte[] buffer, int dataLength)
{
if (buffer == null || buffer.Length <= 0)
return false;
Array.Copy(buffer, 0, _cacheBytes, _index, dataLength);
_index += dataLength;
return true;
}
//...
}
And I create an UnityWebRequest like this:
bytes = new byte[4096]; //As preallocated buffer.
webRequest = UnityWebRequest.Get("url/file.assetbundle");
webRequest.downloadHandler = new CustomDownloadHandler(bytes);
webRequest.SendWebRequest();
But the download speed is same whether I use the downloadhandler or not...