O.K., after some 5 hours of research and experiments, I've got a diagnosis for my Lenovo G560 (dual core P6100 CPU @ 2GHz) - my problem was that it stuck at 46% of the original CPU frequency, with several episodes of 25%, which considerably lowered my ability to use CPU intensive software like Photoshop and games.
The problem was/is overheating and is not related to Windows 7 and it's power management. As far as I understand, the computer BIOS downclocks the CPU multiplier to x7 every time the GPU's temperature rises over 82 degrees Celsius and restores the x15 multiplier when the GPU cools down under 80 d.C. It would probably lower the CPU frequency when other parts of the system overheat, like the CPU cores, but I could not test that, because the GPU always has the highest temperature.
What was actually happening was that HDD-intensive tasks (a combination of Tortiose SVN with huge repos, Cloud Antivirus, ZonAlaram with high traffic and Photoshop using scratch disks) were causing the HDD temperature to stay high all the time, which was causing the GPU to overheat, especially when Photoshop was running and causing the CPU cores to get hotter. As a result, the BIOS never let the multiplier get more than x7, except immediately after the boot up of a cold laptop.
The solution was a) opening and cleaning the laptop (which was not particularly dirty, so this had no effect) b) removing the antivirus c) putting the laptop on this notebook stand Canyon CNP-NS4 (google it)
Now it seems to me that the BIOS uses more often the x15 multiplier and I get no more 500MHz speeds.
What I used for tests:
I found the Turbo.zip tool while reading the following forum thread: http://social.technet.microsoft.com/Forums/en/w7itproperf/thread/b4447bbb-fec4-457f-9ed2-d87965fd6592 (read it for more info on the tool)