Note that if the software makes no use of multiply instructions, the change will be a bad idea, since the cycle time will be increased without any decrease in the number of cycles. Conversely, if the software is nothing but multiply instructions, the change will be a good idea, since one half of the cycles will be eliminated, which will more than pay for a 10% increase in cycle time.
Real software is presumably somewhere in between these two extremes. What fraction of the instructions executed need to be multiplies for the proposed change to be a good one? (Hint: One approach is to define appropriate variables, write equations, and solve the equations.)