Is the time inb's from port 0x84 last constant by
   design/agreement/hardware ?

In theory, reading from a non-existant ISA I/O port should cause an
I/O timeout, which would take 1.25us (10 cycles of an 8MHz clock).  Or
something like that; I don't have ISA specs.  In practice, it seems to
work on many machines, but I wouldn't place any bets.