Thermal throttling
CPU/GPU automatically lowers the frequency when temperature approaches Tjmax (typically 95-105 °C). Protection against physical damage. Symptom: consistent performance for the first 30 sec, then drops — the cooler hasn't kept up.
Modernly aggressive: Intel/AMD CPUs can boost to 5+ GHz as "burst", thermal throttling keeps long-term performance lower. Mobile CPUs and laptop designs live on this. Monitoring: sensors on Linux, HWiNFO on Windows, iStats on Mac. Tuning: better cooling (paste swap, fan swap, undervolt). Datacenter: hot aisle vs cold aisle design, way through tons of cooling water. Laptop CPU underclasses itself (15W TDP but marketed 4.5 GHz peak) — fine for benchmarks, bad for long tasks. Throttle history log: Intel Power Gadget shows exact downclocks.