Your network connection may not be high enough quality to support your selected broadcast quality. Try the following:
You can select quality when you start a broadcast on PS4. Go to [Broadcast Gameplay] > [Quality] to do this.
Update your router firmware. If you are not sure how to do this check the router instruction manual or contact your ISP for assistance.
Your connection can slow down at peak times when a lot of people are online. Try again at a less busy time.