I used to tweak settings in BIOS, but as for now they are all set by defaults, and it probably should set them automatically according to the hardware fine enough.
I will try reinstalling today or the other day, and will see from there what to do. Also, I googled for similar cases, but nothing revealed.
P.S. Even after windows reinstalling and usual procedure of video driver installation and reboot, gpu_lag is still jumping like crazy. There is some improvement though.
P.P.S. Actually, I didn't consider the fact that graphics work far better after reboot, but get slower after complete turning off and on. Strange...