Thanks a lot Exorcet.
You're talking about CloudAspectThreshold. If I'm right, it represents the time, in seconds, between each clouds movement. If you set it to 60, clouds will move only each minute, which make the things easier for the graphic card.
For the benefice of other who would have the same problem than I had, you can see below the modifications that I made to graphics.cfg and that let me have a better frame rate. They have been suggested by isoul in a different discussion. I repeat that I also disable the Windows 7 aero theme before playing, by selecting a theme like high white contrast.
I wonder if someone have made a software with an interface that let you change easily the values of the graphics.cfg and options.lua files, without deleting accidentally an important character like ;
:v: