On the 14th I decided to update the drivers of my AMD video card to version 18.9.1. By evening, I found a drawdown in the frame rate. The next day (today) I started to understand what the problem is. Rearranged RAM modules, reinstalled drivers for the video card (tried versions 18.9.1 and 18.5.1). Nothing helped. But then I began to measure the FPS with the help of Steam and saw with FPS all right. I tried to run /fps. And I had a frame rate drop to 50% of all FPS. Previously, this problem did not arise.
My potato PC:
CPU: Intel Q6600;
RAM: 6 GB
GPU: Sapphire HD7770 GHz Edition.
OS: Windows 10 Pro.
I understand perfectly that the computer is old, but as I described above, in past were no problems with the drawdown.
In the screenshots in the upper left corner you can see the Steam frame counter. Also recorded a small video comparison.