I have the AMD Phenom, six cores.
If only one core is running, it can run faster.
There are even different power management policies that can be adjusted in BIOS.
The clock rates are adjusted dynamically.
I did not look into this for years, because the computer is fast enough for my purposes usually and as long a system works, I dont change it. ;-)
This said there are other undividable or limited ressources like FPU, GPU, memory bandwith, cache size, heap management and as soon as the threads compete for them, the gain of parallel computing becomes lower than expected.