You say to your slow friend: "Damn, you're slower than Stephen Hawking!" And that takes some talent.
Slower Than Hawking
Explanation
Explain Bear
Listen up, buttercup. The joke is that Stephen Hawking, who used a wheelchair because of a disease called ALS, was not able to move fast. You are so slow that you are even slower than Stephen Hawking and you are not even in a wheelchair!