Perhaps try doing this and see if it fixes the high CPU usage.
Enable Mail, Calendar & Feeds News if disabled.
Open the Status bar with Ctrl, Shift, S if closed.
Click the Mail icon. Add it back if removed.
Click on the settings icon.
And click Rebuild Mail Search Database.
There’s also a nuclear option in case it doesn’t help with the high CPU usage. Be careful on which folder you delete.
Simply close the browser and head to this folder and delete the mpognobbkildjkofajifpdfhcoklimli folder.
C:\Users\yourusername\AppData\Local\Vivaldi\UserData\Default\Storage\ext\mpognobbkildjkofajifpdfhcoklimli
Vivaldi mail will simply re-index the mail client.