I cleared the GS_SkyBoxBloom.fx file, and now I get a much better performance with bloom. It doesn't matter, especially for me, that the skybox is not so glowing.
I failed. I tried it again, and this bloom still destroys the picture.
It's curious for me, that a 9800GTX+ can't handle well the bloom effect in this game.
I edited some fx files trying to do something with this issue, but nothing helped. There are two blur effects (vertical and horizontal) attached to the postprocessing while bloom is enabled. These seem to be guilty.