Overheating maybe.. blocked (full of dust/fluff) ventilation grills on the cooler fans and CPU/GPU heatsinks, causing the CPU/GPU to slow down.
I give my PCs and laptops a thorough (strip) dusting every 6 months... you won't believe the shyt (and my place is 'clean') that collects on the cooling fans. My one PC's very sensitive to heat.. and the fan suddenly screams when it gets hot - then I know it's cleaning time.
