I feel you. I also have the same thing when I got elite. Actually I'm a DF1 player, then I stopped from mid-2013 up to July 2016. When I got back, DF1 is now DF elite. I have to adjust because of different mechanics (4 shots to kill a plane instead of 2, bullet drop, WAY MORE shots to kill a tank).
Ok, now back to your issue...It's not only you, I also noticed that it is more difficult to shoot a plane. It's much more difficult to hit the wing as most bullets will just simply pass in between the wings (I'm not sure if this is a consequence of bullet drop), so what I did is to aim at the center of the plane (the propeller). This is much easier for me since your target is larger, but it won't gaurantee a kill. Also the increased plane speeds made it more difficult to aim. You're right; this is not lag. I think zup did this to help beginners since experienced players can easily kill them at a distance. Because of bullet drop, it takes more "calculation" to make a hit, and with increased plane speeds, you can quickly get close to each other, and this will make it a bit more fair for beginners as both become easier targets as distance decreased faster. Also the box is just a guide to make it easier to spot a plane at very long distances.