Poor Safari Performance - Mountain Lion OS X

Question

Since downloading and doing a clean install of Mountain Lion OS X, I have had poor performance with internet browsing. With some of my own basic troubleshooting steps, I have determined its Safari is likely to be the culprit. Below are the scenarios;
1. Ethernet; direct connect to 50 Mbps Modem/Router (Cisco DPC3825)
2. WiFi; connected to signal broadcasted by Airport Extreme (3rd Gen) bridged to Cisco Modem/Router
3. WiFi; connected to signal broadcasted by Cisco Modem/Router
*each scenario was tested on both my ISP's DNS, OpenDNS and Google's DNS.
Safari starts and loads home page (www.apple.com) just fine. Searching or navigating to other sites will require several seconds to minutes of delay, sometimes failing to load. Regardless of this slow performance loading pages; speeds from sites like speedtest.net show my ISP is providing adequate bandwidth, often over 50 Mbps.
I have several other devices on my network that work absolutely fine. ThinkPad T410, two Apple iPhone 5's, two Apple TVs 3rd Gen, xBox 360, Dish Hopper and Joey. All provide quick and speedy navigation. For example, movies load up quick via Netflix apps on both Apple TV and xBox. Safari on iPhone will load much quicker than Safari. Often I'll grab my phone getting tired of waiting for Safari to load and am able to pull it up on my phone quickly.
My current system is early 2009 iMac, 2.66 GHz Core 2 Duo, 8 GB DDR3 memory. Again... a clean install of Mountain Lion.
Also, I loaded the latest FireFox and browsing is perfect. Fast and very responsive. However, using FireFox greatly degrades the Apple iCloud experience and I would greatly prefer to use Safari.
Any ideas on what I can try next to get my iMac back in tip top shape?

Answer 1

I was able to make some configuration changes on my Cisco DPC3825 router, my speed is much better. I've tested it for a couple weeks now and have not had any lag in browsing. Here is what I changed;
1. Log into the Cisco Router's control panel.
2. Security > Firewall > Disable SPI Firewall Protection (and any filters that were selected by default or your ISP)
3. Access Restrictions > Basic Rules > removed a site in the blocked domain list
4. I then used Open DNS DNS servers and managed security using Open DNS.
Please take this information lightly, I am not a network expert. But these steps allowed me to get over the problems I listed above. I'm getting very fast speeds and just about 0 lag when browsing internet.
Hope this helps someone.