Effective Address calculation time on 8086/8088
Gracious reply from Stephen Morse (designer of the 8086)...


  Boy, you are really asking a question from my deep distant past.  The
  answer obviously has to do with the way the addressing modes were
  micro-coded, and the person who wrote the microcode (Jim McKevitt) is
  no longer alive.  So I don't know how you can get an authoritative
  answer.


A definitive answer may have to wait for someone to reverse engineer the silicon...