No priority boarding at IST
 

Separate bus is only from the plane. I guess it is due to current infrastructure constraints. Maybe it would change with the new airport?

It's difficult to manage premium-class bussing to the aircraft. From the plane is simple because you have all the passengers nicely grouped and waiting.

For departures ,there'll always be one set of J-class passengers determined to be first on board, loitering around the gate for 10 minutes before boarding begins; others turn up during the boarding process, while a further group enjoy being among the last to board. Airlines (and the airports) don't want the inefficiency of running half empty busses in shuttles out to the stand - so the solution adopted at most places is to send all classes out on the same bus (as at Heathrow), or ask premium passenger to wait until the end of the boarding process so they can travel out together on "premium" transport.

This second strategy is wildly unpopular among the group who want/need to be the first on board, so it's seldom employed (it's something Qatar does for remote departures, though they keep tabs on the premium number checked into the gate area and send them off when they have the full complement, or a bus load of them).

A further variation is to use busses with a premium class section, the doors of which can be opened at the aircraft before the doors of the economy-class section. This isn't very efficient and I've seen it only among low-cost airlines anxious to preserve the "speedy boarding" experience that passengers have explicitly paid for,