Most likely the problem is the provider. Basically everyone has a high speed connection more than capable of sending multiple HD streams to a house full of users. IPTV really doesn’t need that great of a connection, even my cell phone can pull HD channels without breaking a sweat (my 10gb data plan not so much though).
Instructions unclear, my Shield has poor WiFi in the microwave
Make sure your connection is up to the task. Run speed tests on your wifi, or wire it if possible. Are you trying to run content that your speeds can’t handle?
Make sure your equipment is up to the task. What device are you using?
Make sure your software is up to the task? What are you using?
Finally, maybe not last, but is your service experiencing issues? Do they have a Discord for support? Are you using more than one service and are they both experiencing issues?
If you find an answer on how to resolve IPTV buffering please let us all know.
Edit: also, if you’re in the UK your isp could be throttling you. Use a vpn.