I'm not sure what to tell you. I'm running Iceweasel pretty much vanilla, with several add-ons, and it's not slow for me. Running in private mode will slow it down, I think, as will IPv6, but otherwise I've seen no problems. Pages load about as fast as can be expected based on my internet connection speed. At work, where we barely have a usable DSL connection, and several computers connected, it's obviously slower than at home with a good connection and no other computers, but all that is expected. On my home network, pages load almost immediately.
One thing I've done is replace the DNS lookup with OpenDNS, and uncheck the boxes for requiring IPv4 and IPv6 to complete the connection. I'm not sure how much that helps, but using OpenDNS seems to make page loading snappier.
Asus eee-pc 900 w/ 32GB SSD.