Streaming to TV

Anything can cause this.

How fast is your Internet connection?

Do you have any other users using devices on your home WiFi network?

Are you trying to stream video from your iPhone to iPad to TV using the iPhone's cellular WiFi hotspot feature using your cellular data?

Data traffic on the Internet or over cellular can cause issues, too!

Try AirPlay-ing the video directly from the iPhone instead of using the iPad as an intermediary device.

See if you get better results.

Best of Luck to You!