This is always one of my first steps, to enter an SD-Card and set it as the default location to store the photos. It has the advantage, that if the phone should get damaged, the pictures are still available.
The location you can set in the settings of the stock camera app, it is called “Speicherort” in german, which translates to something like “storage location”.
About the storage speed, I never had problems with a slow storage, but you should buy an SD-Card with good specifications like U3 / V30 or faster. It isn’t worth to save on the SD-Card.