This is not a new thing, but it needs to be posted about again and again until something is done about it.

For those lucky few who haven't noticed yet:

DESYNC:
When a red or yellow chain appears on your screen, shots you fire will not register as hits until several long seconds afterwards, items you pick up will not go into your inventory, and getting in/out of vehicles takes forever. Enemies will sometimes teleport, change speed/direction randomly, or kill you without visibly having a weapon equipped, making a gunshot noise, or even facing in your direction.
Even when the chain isn't present, there is still some desync-usually most noticable in close quarters combat. Since shots take about a second to register in most (normal) circumstances, you can put a clip into a players back, they have time to turn around and shoot you dead before dying themselves. This is game breaking.

MULTIPLAYER FPS::
In ARMA games, client FPS (the frames per second you get) is capped by the server's FPS. The problem is, the game taxes servers so much not a single server can get decent FPS - so even people who get 60+ fps in singleplayer with max graphics ALWAYS get less than 30 fps on a multiplayer server, regardless of how many other people are playing. This is made worse by AI, number of vehicles, and number of players joined... but even on a one player server with no AI and no vehicles FPS is still less than 30.