I say hello to you again.
I completely uninstalled Comodo 6, I wiped it, and then I installed it again.
The situation seemed a little better but essentially it was not changed.
Bad, very bad.
However I reached a great result anyway.
In fact I realized that some operations (for example searching files by Search of Windows Explorer) are strongly slowed down when they deal with Zip files. I suspected that Comodo scans those files thus causing a great waste of time.
Therefore I deactivated the real-time scan of compressed files (see "Decompress and scan archive files of extension(s)" in http://help.comodo.com/topic-72-1-451-4756-Real-time-Scanner-Settings.html
Since then the situation is greatly improved. Security is not compromised: in fact Comodo no longer scans Zip files in real-time, but it scans them anyway when the user extracts the inner files. Sooner or later a real-time scan is always performed.
Therefore the presence of Comodo 6 is bearable now. I keep it.