This is one of those cases where it would really make make sense to do all the position computations at the client side and merely provide "correction packets" to adjust the local/client-side extrapolation in fgfs, otherwise we'll be creating crazy amount of traffic unfortunately.
You really don't want to transmit position at all, you want to transmit derivative information. The correction would directly refer to the various velocities, i.e.:
* speed up to 280 kt, change vertical speed to 2000 fpm and turn with a rate of 30 deg/minute
would completely specify what the AI system has to do to get to the next known position, even if that position is a minute away (just instead of position interpolation, you calculate the velocities which will get you there). Basically, all you need is a single packet per minute for each plane if the AI system handles the actual motions client-side.
This is going to remain an issue anyhow: Whenever "real" data comes in, the "projected" (computed) flight path will need to be updated, even though it has already been used for up to 60 seconds by then. So aircraft will definitely "teleport" for the time being ...
No, it's not an issue if you accept a 1 minute lag, because then you know the future.
It's a simple progression:
* with one point (no lag) you can create a staircase curve 'close' to the real one which is discontinuous
* with two points (60 seconds lag) you can compute a derivative and make a continuous interpolation with discontinuous 1st derivatives
* with three points (120 seconds lag) you can compute a 2nd derivative and make the 1st derivatives continuous (I assume we don't really want that, as it then requires to also transmit accelerations as part of the MP packet)