“Casting” actually has a minor impact that may add up if you do it too much (inside Tick events of many actors, etc).
They could impact your code performance if abused, it’s worth watching this series if you didn’t already:
https://www.unrealengine.com/en-US/e…depth—part-1
https://www.unrealengine.com/en-US/e…depth—part-2