On performance "Given" vs "When" it's actually the same.
What we should be looking here is what is the actual test.
By definition:
- The **given** part describes the state of the world before you begin the behavior you're specifying in this scenario. You can think of it as the pre-conditions to the test.
- The **when** section is that behavior that you're specifying.
- Finally the **then** section describes the changes you expect due to the specified behavior.

(source: [Martin Fowler - GivenWhenThen](http://martinfowler.com/bliki/GivenWhenThen.html))

So here the action that we want to test is the "go to"/"open" page action.
Can also see: [Cucumber](https://github.com/cucumber/cucumber/wiki/Given-When-Then#given)