This may be a result of the Tab casting quality setting.
Check the settings by following the steps below:
- Click the Cast icon in the Chrome browser.
- Click Options.
- Check the Tab casting quality setting.
Also, the performance of the cast may be restricted depending on the computer Operating System. For more information about Operating System of computers, refer to Your casting performance may be limited of the Chromecast Help website at https://support.google.com/chromecast/answer/3210065?hl.