It's part of normal operation for any CPU/GPU released in last 10 or so years.
There is a reason why intel advertises a regular and "Boost" clock speeds. Boost clock is not sustainable without heavy duty cooling and the CPU will throttle down once it's heated up.
I don't know if it still holds, but some older systems also had a third category of throttling in between the normal speed range and an emergency power off. They would stutter badly as if they were applying some duty cycle and alternating between a suspended state and a slowly clocked state. This would show up as really frustrating IO buffering and UI latency, similar to how a machine might behave if it were stuck in a virtual-memory swapping storm and processes would freeze/unfreeze sporadically.