All I know is since I went from snow leopard to mountain lion my macbook pro 2011 13" 2.3GHZ i 5 2410M is slow to load web pages.I am not an expert and that is all I can say.
In my experience, since your upgrade from Snow Leopard to Mountain Lion, your system does not have enough RAM and this is causing it to run more slowly. Upgrading to more RAM should speed things up substantially.
Snow Leopard, when booted up to the Finder, takes approximately 2GB of RAM to run (that is if no apps are loaded and running). Mountain Lion, however, takes approximately 3.5GB of RAM in the same situation. When you start running apps, including Safari or any web browser, the amount of memory needed by the system goes considerably above these amounts. If your Mac has less than 4GB of physical RAM, then Mountain Lion is going to use the "swap file" to constantly write pages in and out of RAM and to the hard drive, which is going to slow down your system operation considerably.
If your system will support it, it will run much faster if you can upgrade with more RAM. According to this chart, your model of Mac can accommodate 8GB, 12GB or 16GB of RAM (confirm the exact model number to be sure), so you should consider upgrading.