do you play in Chrome or using the Chromecast Ultra? Maybe you want to check if your wifi is fast enough that the location of your device.
If you are on a phone or Chrome, please use the Stadia speed test to verify your wifi performance 🙂
Hopefully this will help with your issues, but you can always also try to contact Stadia support to receive more tips or more troubleshooting steps.
You mentioned you're using the app on mobile, have you tried it out on Chrome Browser or on your TV with the CCU, just to see if you have the same issues there and to isolate if it's a problem with your device?