If you have not long re-installed your OS then it may just be a network problem such as a lot of people on the web causing it to be jammed.
Give it a while and it should sort itself out.
As for your question, is it the OS then the OS won't cause the problem and I doubt that it's your firewall, you can go into your firewall options and see if port 80 is enabled that may speed things up.
It may also just be the sites that are experiencing trouble, so why don't you go to www.speedtest.net
and test your broadband speed.