The incorrect number of CPUs or Cores are being reported by PerformanceTest
If PerformanceTest doesn't detect the correct number of CPUs in the system or the correct number of cores, then it is likely that the CPU(s) will not be fully loaded during the CPU benchmark, and you'll get poor CPU performance results.
If you are running PerformanceTest then check then check what is reported on the "System" and verify the CPU and core count is correct for your CPU.
The only case we are aware of it not being correct (as of Mar 2011) is when there is a problem in the BIOS configuration.
In some BIOS setup screens you can find a value called something like,
"Max CPUID Value Limit" or "Maximum CPUID Input Value BIOS" or "Limit Maximum CPUID to 3", etc...
This CPUID limit setting needs to be disabled if you are running XP or later.
The technical reason for this is as follows.
CPUID is a low level machine code command that can be executed on the CPU to gather information about the CPU. This information includes details like the make and model of the CPU, the features it supports, cache configuration and the number of cores available.
New CPUs make much more information available about themselves than old CPUs.
Full details for Intel CPUs can be found here,
When Windows boots it queries the CPU to see what level of information is supported (what numbers can be set in the CPU's EAX register). Old operating systems like Windows 98 didn't support a value greater than 3. So to support new CPUs with these old operating system the BIOS developers included this option.
Some programs like PerformanceTest also use the same CPUID instruction to detect the available CPUs and Cores in the machine.
So having this value set wrong can result in wrong system information being reported for newer CPUs and thus lead to performance problems and other strange behavior.
As an example of what might be (incorrectly) reported in PerformanceTest, if this BIOS setting in incorrectly enabled. A Intel Core i7 950 might be reported as having 1 core instead of 4 (or 8 with hyper threading).