How can I measure sustained CPU performance on my PC?

OCCT is a good free tool for this because it can apply a continuous CPU load while recording temperatures, clock speeds, power usage and throttling.

A simple test would be:

1. Connect the PC to power, close unnecessary programs and use the power mode you normally use.
2. Let the computer sit idle for about 10 minutes so it starts from a stable temperature.
3. Open OCCT, select its CPU test and set the duration to 30 or 60 minutes.
4. Watch the CPU temperature, effective clock speed, package power and any thermal or power-limit warnings.
5. Compare the readings from the first five minutes with those from the final ten minutes.
6. Stop the test if the computer shuts down, reports errors, or reaches abnormal temperatures and cannot reduce them.

A small clock-speed drop after the initial boost is normal. The important question is whether the speed settles at a stable level or continues falling as the computer gets hotter.

For a more realistic video-encoding test, you can also encode the same long video with HandBrake while monitoring the CPU. Record the average encoding speed near the beginning and end. For gaming, use a game with a repeatable built-in benchmark, since a CPU-only stress test does not include GPU and game-engine limitations.

Keep the room temperature, cooling setup and power settings the same when comparing results. Otherwise, the numbers will not be directly comparable.