Google services freeze
If you're having this problem only with Vivaldi then it may just be an issue with that specific version. Also, if you are using an older version of OS X, then the problem may be the system may not be able to handle a certain webpage or you have a large amount of tabs open and Vivaldi freezes.
Tips for the future: You may want to update Vivaldi to the latest snapshot OR re-download Vivaldi. If both of those methods don't work, then I would advise trying to cut back on the memory usage on your Mac.
Hope this works for you!