In my experience, when the bucket has lots of objects, using the web interface or gsutil alone is not the best way.
What I did, was that I added a life cycle rule, to have Google delete all the object in the bucket.
[![Add lifecycle rule under the "Bucket Lock" tab][1]][1]
[![Now have Google delete objects][2]][2]
Then, coming back after a day, the bucket can easily be deleted.

[1]: https://i.stack.imgur.com/bvwtX.png [2]: https://i.stack.imgur.com/zMEUm.png

In my experience, when the bucket has lots of objects, using the web interface or gsutil alone is not the best way.
What I did, was that I added a life cycle rule, to have Google delete all the object in the bucket.
[![Add lifecycle rule under the "Bucket Lock" tab][1]][1]
[![Now have Google delete objects][2]][2]
Then, coming back after a day, the bucket can easily be deleted.

[1]: https://i.stack.imgur.com/bvwtX.png [2]: https://i.stack.imgur.com/zMEUm.png