Thanks Leo : i had already tried the things explained there :
No specific files or folders .. they can contain pictures or music or whatever else
No alcohol, disabled Use Shell Image extraction, disabled all viewers plugins
But then i have read this one :
Thumbnail caching have been purged and disabled
I had no mapped/network drive
I have turned on Generic icons for all folders
When i have done that, it worked better : the problem would not occur on all folders.. still strange ..
But then i tried W0lfdale's suggestion [resource.dopus.com/t/slow-response-on-all-listers/4606/3)
and found Mozy was the bad one .. it seems the new version of Mozy Backup is trying to check each file of each folder to tell if it has been backed up or not by changing the icon .. quite mad idea because it's very slow !
I have disabled it and it works real fine .. In Mozy, you need to check "Disable icon overlay in explorer"
Thanks for your FAQ Leo !