I finally tried to post a bug in Mandriva's bugzilla, and got immediately following answer :
That's because the floppy is now mounted with 'sync' as default...
If you want the speed back, change in /etc/fstab the line regarding /mnt/floppy from ...,sync,... to ...,dirsync,...
You will only get a little bit longer umount as the kernel flushed the writes to the floppy... but the overall total speed is faster that way...
I tried this solution, and it works : now writing to floppies is really faster, BUT, it's very important not to remove a floppy without unmounting it first. In fact, without former supermount it's now very important to perform systematically mount/unmount. I noticed that mount/unmount options exist at user's level with a right click on the floppy icon in the new devices window.
;)
_