Thanks Mike,
Maybe your explanation about the SAN was right because now whenever I issue my statement, the operation is taking about 5 minutes.
I did several tests by restarting the dataserver to clean the cache and re-execute the SQL statement and now it's quite fast.
As long as I don't know what's behind my FileSystem and that I can't have the control over it. I'll have to leave with it
Anyway, thanks all for your help, it was very helpfull and usefull to read.