[Kib]

If the network speeds are ok, then what could be the issue is the amount of buffering XBMC uses.
The default buffer is 5 MB, but this can be enlarged by using advancedsettings.xml

Check Advancedsettings.xml#.3Csamba.3E (wiki) and Advancedsettings.xml#.3Cnetwork.3E (wiki)

Code:

<cachemembuffersize>5242880</cachemembuffersize><!-- number of bytes used for buffering streams ahead in memory
    XBMC will not buffer ahead more than this. WARNING: for the bytes set here, XBMC will consume 3x the amount of RAM
    When set to 0 the cache will be written to disk instead of RAM, as of v12 Frodo -->