I can confirm this, after half a day of not being able to use my favourite browser i found this thread, thanks to OP i found the culprit. Was so happy to finally have my team's shared calendars in Vivaldi and then noticed a constant 100% load from Vivaldi (noticed bc my battery drained way faster than usual).
Tested with stable v5.4 and then also with snapshot v5.5, both have the same problem... as soon as the shared calendars are unticked here https://calendar.google.com/calendar/u/0/syncselect (don't forget to refresh the calendars in vivaldi) CPU usage goes back to normal levels...
It's a real pity, hope this can get fixed soon @eggert
EDIT: also noticed, this is in a forum where it might not get enough attention, I'm on an M1 Macbook (which is of course ARM as well...), but this will affect a lot of people, maybe there is an issue in your internal bug-tracker already for this? an update would be appreciated.