thanks, it seems that disabling composite (*), a bit of triple buffering and caching completely resolved the issue; actually now it seems to work better in linux than in windows. i'm keeping an eye on it but seems fine.
thank you again.
(*) a pity though; the ability of zooming the desktop could have been cool and useful too, on such a small screen (netbook)
CORRECTION: disabling composite extension leads to the best results, but even with triple + composite-on the results are not perfect but okay-ish