Question

Why is the rate determining step the slowest step?

Answer 1

If A + B ==> C fast

and C + D = E slow
and E + F = fast
step 1 is essentially instantaneous if it is fast and so is step 3. But step 3 can't take place any faster than step 2 so step 2 determines how fast the whole reaction is since anything past step 2 can't take place faster than 2.