A 3bit adder can only calculate its result after the carry bit of the lower digits are known. The execution time of a 3bit ripplecarry adder is therefore 5 × τ = 300 ns! (see illustration).
Each additional digit must again wait for the result of all the lower digits. The execution time of a ripplecarry adder of two 8bit numbers would already be 900 ns!
Modern microprocessors have arithmetic logic units that handle 64bit numbers. An addition of two 64bit numbers with a ripple addercarry would take an eternity compared to other execution times of the microprocessor. Therefore, no ripplecarry adders are used in modern microprocessors, but socalled carrylookahead adders... 
