There are several major bugs that have been around since the release of the game, and this is still one of them.
Originally, this would most quickly occur when you flew near a formation of 3+ 2 engine German (player) bombers. Don't ask me why it didn't affect the Blenhiem, but it just didn't. It only affected the Ju88, Bf110, and He111 (I didn't have the opportunity to fly the BR20 in formation enough to know if it affected it or not). This would also occur in dogfights with 4-5 player flown single-seat planes, but it didn't always happen.
At any rate, the game would slowly drop FPS until the game slowed to a .000001 fps and then the game would crash and we would get a launcher.exe error in windows.
About 3 or 4 patches ago, this was changed. Now we do not get the launcher.exe error, but if you try to fly a large bomber group, someone wil have an FPS drop below 1 fps and stays there. About 2 patches ago we tried this with a group of ~7 He111's and we lost about 3-4 players because of launcher crashes or FPS drops. Also, we still found this in dogfights with 4-5 planes.
Anyhow, the video below shows the same FPS drop I would normally get when flying in a tight bomber formation. There may have been a Ju88 in view distance, but I did not see it. I do know that there was a dogfight roughly 5-6km away from me when I filmed this video.
I only notice this type of bug in multiplayer, I have never been able to re-create it in singleplayer. I've also never noticed it when diving on AI controlled planes in multiplayer. I've attacked a formation of over 12 He111 AI planes with a steady ~40fps.
VIDEO:
-Another major bug that has been around since release is 'ghosts.' We believe it has something to do with de-spawning AI flights. When the game was first released, 'ghosts' were actually pilots/crew members sitting in mid -air where an AI flight had passed by. Now, there are no longer people sitting in mid air, but we still render the distance render dot of an object. When you approach this (what used to be an AI flight) there is nothing there. It is just a distance dot. When you get close, it disappears, but after you pass by and look back, the dot is there again at a distance. To my knowledge, this only occurs in multiplayer.
-The last one is just a graphical problem. Running at a high resolution is a hindrance in the game because when you see a contact at a distance and zoom in, it appears smaller (because you are rendering 1 pixel). However, if you are rendering at a low resolution, you can easily see contacts at a distance. The worst is that bombers, boats, and balloons seem to explode from a small do to a 3d object. Also, clouds seem to magically appear at a distance of ~5000m away. Rendering needs some work. And something especially needs to be done for ground object rendering. It is impossible for player bombers to see and line up on a target for a level bomb because they do not render ground objects until they are within 5000m. So for a bomber trying to hit a factory complex at 5000m, you are literally over the target by the time you can render it.
Because of the scale of the game, I would suggest allowing the mission builder to designate objects that will forcibly render at a distance.