You can't really compare just the number of cores in different processor architectures (i.e. Celeron vs. Atom) and draw meaningful conclusions about how they will perform against each other without comparing other specs as well. Atom has four cores but each individual core will not be nearly as fast as one of the cores on a Celeron. The Atom architecture is ultra-low-voltage which saves power but has reduced performance; a single Atom core is about half the speed of a single Pentium M core.
Also notice that the VPN throughput is actually higher on the 3100/3200 vs. the 5100/5200, this is because the 3100/3200 supports the AES-NI processor extensions whereas the 5100/5200 models do not. AES-NI support resumes with the 5600 model and higher, except for the Power-1 9000/11000 series and 12200. The 5800 and higher (except for the Power-1 9000/11000 series and 12200) also support SMT/Hyperthreading.
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