Have you tried another browser to see if it is as slow as FF. Is your computer the only one connected to your internet, (ie, are there any other computers sharing your connection). If so have they been scanned for malware, and viruses. I would scan your computer, and any others on your network with malwarebytes:
http://www.malwarebytes.org/mbam.php
Your bookmarks will have nothing to do with the slow down, I have about 700 bookmarks, and firefox works as advertised.