Here is a video of a real Sinclair ZX80 computer playing Space Invaders at the Vintage Computer Festival 2010 at Bletchley Park. You can see that the screen does not flicker when a key is pressed.
This computer was on “The Centre for Computing History” display. See http://www.computinghistory.org.uk
It was fantastic to see all the old Sinclair machines running on the Centre for Computing History’s display 🙂