WHY IT WORKS
The slowdown is heat, not the hardware.
When a processor crosses its thermal limit it reduces its own clock speed to protect itself. Nothing breaks and nothing warns you — the machine simply gets slower under exactly the loads you bought it for. Dropping the intake temperature by a few degrees is often enough to keep the clocks where they belong, which is why a pad of three fans can feel like an upgrade.
Keep it cool and it keeps its speed.