Kahana timeline

So, when the helicopter with Sayid and Desmond first went from the island to the Kahana, they left at night and arrived a mid-day. So they went back about twelve hours in time. OK, I can accept that.

Though, journeys are time affected (helicopter, Dan's rocket thing) but communications arn't, right? Is this because commuications are carried in waves that travel much faster than (say) helicopters? Is there a complete time delay? Light being the speed that it is, there would be an unnoticeable delay but could still be a delay.

The time difference is again evident in the Dr's death, except that the difference is much greater. So, time is speeding up on the island, relatively, yes?

I should clarify that I'm talking about the final few episodes of series 4.

The simple fact is.... I'm confused.

P.S. sorry for the random rambling nature of this post but it does reflect how my mind is working