Slowest – Continental Drift
The average relative speed of continental drift is about 10mm per year.
At one time all the current continents were joined together in one supercontinent called Pangaea but over Earth’s 4.5 billion year history this was broken apart into the continents we know today.
Fastest – The Speed of Light
Light is so fast that it wasn’t until 1926 and the work of A.A. Michelson that we had a good idea of just how fast it is. Light could travel the distance from London to Sydney (13477 miles) and back seven times in one second.
Albert Einstein’s Special Theory of Relativity shows that the speed of light in a vacuum is the absolute fastest it is possible to travel. No matter how much energy you have, no matter how big an engine you create, no matter where you are, it simply isn’t possible: if the Universe really is full of aliens then it’s going to be a very, very long time before we meet.