Physical quantities must have a unit, a number without a unit is meaningless. A colleague of mine used to highlight this fact an every opportunity he could with the above question. So below the question is answered
First convert a centimeter per second into a standard SI unit, which for speed/velocity is meters per second. Thus 1 millimeter per second is 0.001 meters per second (0.001 ms-1)
So how to convert a furlong per fornight into meters per second?
1 fornight night is two weeks, two weeks is 14 days, 14 days in seconds is 14days x 24hours X 60minutes X 60seconds = 1209600 seconds
1 furlong is an eigth of mile, 1 mile is 1609.34 meters. Hence a furlong is 1609.34/8 = 201.168 meters
Therefore a furlong per fornight is 0.00017ms-1
Thus, from the above, it can clearly be seen that a millimeter per second is significantly faster (nearly 6 times) than a furlong per fortnight!