Verified using sequences from the Online Encyclopedia of Integer Sequences, specifically:
Decimal expansion of golden ratio phi (or tau) = (1 + sqrt(5))/2
F(F(n)), where F is a Fibonacci number
L(L(n)), where L(n) are Lucas numbers
There is a slight but noticeable slowdown that starts around n = 30 on my MacBook Air.
At n = 40, you have time to go pour and dress a cup of coffee.
If anyone knows of a more efficient way (memory/performance) to handle this, I'm curious to hear it.