In the scrum guide it says: "The Scrum Team may refine these items during this process, which increases understanding and confidence."

The design grows as more is learnt.

Pair programming means that two developers are coding together and performing live peer review. This will enhance the effectivity as it leads to increased code quality. Regardless of your opinion, when answering Question:s about pair programming, know that it's considered a really good practice for PSD. And in case you are wondering, the instructor of the course is also a big fan.

The scrum guide says: "The purpose of the Sprint Review is to inspect the outcome of the Sprint and determine future adaptations. "

It's best to do it often during development so that problems can be detected early. Perfomance testing on production will disturb the production environment.