Let's look at the apple transition from powerpc to intel chips. Geekbench has a reasonable number that we can look at to compare against, so let's use that. http://browser.primatelabs.com/mac-benchmarks
We can see that the one of the late 2005 Power Mac G5 had 2 2.3 Ghz cores. This garnered a score of 2088.
About a year later, a mac mini was on the market using an Intel chip. In this case, the Intel Core Duo T2300 which runs at 1.66 Mhz with 2 cores. This received a very similar score of 2128, even though it is running considerably slower.
Later on in the years, I can come up with a Mac Mini (Mid 2011) that has an I5 running at 2.3 Ghz (The same as the original PPC we're comparing to). However, this gets a score of 5818, more than double the original PPC I picked.
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My only point here is that it is VERY difficult to use processor speed as a tool to compare processors. It's even difficult to use it to compare a Xeon vs an I5, even though they both use the same architecture and are made by the same company.