Things like SpeedStep, self temperature monitoring, CPU load throttling and core hibernation are enough for me to rule out the "Monitoring system performance" option as being the one.
But storing data... CPUs do that, there's nothing else in the system that does that operation more than a CPU. They have to store data in registers, L1/L2/L3 cache and RAM all the freaking time!
Those aren't monitoring 'system performance' though, they're technology that slows the CPU down when it's not in use or getting too hot etc. At best you could say they are monitoring one component's performance but I wouldn't even say it's doing that really.