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2.4 for base 2010. 2.0 for base 2011. Why is this?

Various reasons:

Slower clock-> Less power -> Less battery usage.
4 cores now instead of 2.

Don't worry too much about clock speed, (I had a 3GHz cpu about 6-7 years ago) it WILL be faster.
 
The processors are more efficient, and have more cores. They can get more done per cycle than older processors, so typically the lower Ghz here don't mean much.

Simplified explanation of course. I am sure someone will give the detailed one very shortly. :)
 
Let me explain this mathematically:
2,4ghz x 2cores = 4,8ghz.
2,0ghz x 4 cores = 8,0ghz.
Not exactly like that, because not all programs (actually very few do) use all the cores.

To clarify, it is as if you have the combined power of all these cores running together in parallel.

But even at single tasks that cannot take advantages of the cores, typically newer chips can get more work done. For example, the new 2.0Ghz i5 chip is way faster than my 2.5GH C2D chip.

It is better to have more cores than higher Ghz in most cases, as you have different applications running on your system asking for CPU time.

I believe that new chips (the older ones might have had them too) is using Intel's Turbo boos technology, which can boost the speed of a single core more than the stated Ghz if the other cores are not in use.
 
2.0 is boring. On single threaded applications (Read: the majority) the proc will turbo boost up to to like 3.1 or so GHz. Less turbo boost available as the number of cores in use goes up .

Gist: It's faster than the 2010 model.
 
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