3G

Okay Asoka, is it possible your handset isn't using "Auto" for selecting the mobile data network mode? 

Perhaps your handset is set to select 3G - alternatively, there may only be 3G network coverage available at your location in France. 

Asoka  If a network is busy, I feel that they will often prioritise their own customers over people roaming from another country. To be fair, I don't blame them because I would expect that as a direct customer of a network they would give me priority. (Might that be the reason why 3 network customers get the better speeds over Smarty customers?)