Hmm, what kind of mods did ye download, exactly? Some may change the way FH loads meshes, like moving leaves on bushes and trees, which may cause more lag and y'er computer may not be able to handle it.
Also, did ye download any particles? If ye did or even if ye didn't, check y'er particles folder found in media and check f'er any files named with a "_/any number/" at the end of it. If ye do 'ave some of these, delete the numbered ones and try to load y'er game 'gain.
Hope this helps ye deary!