I think the delay on slaves assisting is ok, no?
Because:
  1. Focus targets "Mob A"
  2. Server refreshs "Focus Targeted Mob A"
  3. Slaves requests assistance to Focus
  4. Server sends "He is targeting Mob A"
  5. Slaves targets Mob A
I have the little delay on all my toons, so sometimes I need to press the DPS key 2, 3, 5 times until they target/start casting the target.
If this isn't normal, then I will keep looking this thread for a solution.

EDITED: I use X-Perl unit-frames, it shows real time party/raid:
targets, target target, target target target, focus target, focus target target.

And WOW your computer is the same as mine :P (Q6600, 4GB, 9600gt).