FPS loss when engine started

[xcom]

So far I've tested this only in the P-51D...

Whenever I'm on the ground, everything off and have planes flying around me... I get about 60FPS (also when i'm in the cockpit), right as I turn the engine on the FPS goes down to around 25-30.

 

Can anyone explain why is this happening? or maybe a way to check what is the problem?

Btw, I also get huge FPS loss whenever I look at gun tracers, around 10-15 frames loss.

 

my system -

Gigabyte Z68mx

I5 2500K OC 4.2GHZ.

4GB Ram DDR3

Radeon HD 6870 - 1GB.

 

Card settings -

AA & AF - application controled.

Tesselation - application controled.

Catalyst AI - disabled.

Vsync - off

Tbuffering - off

AA mode - Multi-sample.

 

Game settings -

AA - X8 (tried with X4 same deal)

cockpit shadows off.

TSSA - off

Tree shadows - off

Heat blur - off

Visib range - high

Textures - high

Scenes - medium

Shadows - high

Water - low

Resolution - 1680X1050.

Windowed mode - off

HDR - off

 

Hope I didn't forget anything, appriciate any help this is really getting frustrating!