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Have a look at Activity Monitor (Applications / Utilities /) and select All Processes and sort by CPU to see what the culprit may be.
Also check the "System Memory" tab to see what your "Page ins:", "Page outs:" and "Swap used:" are.
Adobe Flash Player is CPU intensive, thus more power is needed to feed the CPU, which results in higher temperatures and often higher fan speed(s) on mobile computers. As more power is used due to Flash, the battery life of mobile computers is shortened by a good bit.
To check, if Adobe Flash Player is responsible for less battery time, go to Applications / Utilities / Activity Monitor and select to show ALL PROCESSES and sort by CPU.
There are a variety of Flash blockers for all the four major browsers available.
A Flash blocker does what it says, it blocks Flash content, but via a click on the marked Flash object, the Flash object can be activated and used.
Safari: ClickToFlash, for which there is an extension and a plug-in; CTF allows you to download YouTube and other kinds of MPEG-4 encoded video too.