What is the slowest step in HVZ reaction?

#1

what is the slowest step in HVZ reaction

#2

The step when negative charge is formed on the adjacent ch2 and Br is attached on it must be the slowest most probably, though I'm not sure

#3

In this reaction, the attack of PBr3 is the slowest step,so this help in determining the rate of reactions.
Screenshot_20190824-020641