Renew Subscription fail

Have you setup an event filter for WebHook expiry events? 

/restapi/v1.0/subscription/~?threshold=XXX&interval=YYY

Using this event filter you can manage the threshold and the interval for the events and you can re-subscribe later. 

For more information on how to receive expiry events, please take a look at our 

QuickStart Guide for Subscription using Webhooks

So what I suggest is one of:  remove the need to renew subscription or raise the 1000ms limit at least

The 1000ms requirement should be for general response, e.g. creating a webhook (verification) and events. It's interesting that you are only seeing this during subscription renewal. If this is the case, you can proactively renew the subscription since you know the expiration time. When you see a verification header you can have your server respond immediately without calling the other API endpoint.

We are looking into non-expiring webhooks but the above may be able to solve your situation now.

Finally. My test doesn't seem to give result. Always have the 
{
    "errorCode": "SUB-521",
    "message": "WebHook is not reachable",
    "errors": [
        {
            "errorCode": "SUB-521",
            "message": "WebHook is not reachable"
        }
    ]
}
error and this is exactly the same thing when I register the webhook the first time and this is not fast enough for ringcentral. Need to retry a lot of time until this work. But renewing make the webhook deleted if fail and I need to register it again...