I have an gtx 690 running bf4 at 2560 res and tryed ultra, but it give huge fps drops when large events happen, e.g. tank blows up plane crashes buildings falls down.
Any help would be great
That card at the resolution and those settings is not realistic, try high no AA. The jump for ultra to high isn't that bad and it gives you quite a bit of fps
It's due to the massive VRAM bottleneck. The 690 only has 2GB of effective vram so really struggles with ultra textures are 4x MSAA. Turn those down and you should get much better fps.
As said above you can also try turning down things to high, there is little visual difference.