resolved: the problem lies on Google DNS. Facebook's images are on akamaihd servers that are selected geographically as closest to you. this is done using your DNS. if you use Google's DNS (8.8.8.8) it gives akamaihd the wrong location (maybe India.. and I am in Italy) and as a result all Facebook contents are so slow to display that it becomes unusable.
Solution is changing your DNS to your provider's.

12.17
I however found out that the problem is plaguing millions of people. it's related to akamaihd servers hosting facebook images: they take ages (sometimes forever) to provide Facebook content (so they can't be blocked via hosts file)
I am however puzzled because I run 3 other browsers and this happens only using Opera

Facebook takes AGES to load on Opera Portable (while in chrome portable or firefox portable it goes fast and smooth). sometimes the pages never load fully even after hours. what's the problem? should I enable disk cache (I didn't in chrome or firefox and they work wonderfully).