Webhooks will always have some delay before sending notifications to ensure consistency between all cache servers of the underlying API endpoint. For some topics, such as stream change, there are additional delays on things like a stream going down so that if it’s just a momentary connection drop that is recovered from it wont send out a notification.
up to 3 minutes is common in most cases, with a stream doing down often taking around 5 minutes.
Twitch have said they’re working to reduce the delay, so expect some improvements in the future but no ETA on when.