Dust and airflow
Dust buildup inside the fan and vents is the most common reason laptops slow down and get hot under normal use. A few years without a clean and airflow can drop enough to affect performance.
Thermal paste
The paste between the processor and its heatsink dries out over time. Once it does, the laptop runs hotter than it should even right after cleaning, and renewing it is usually the fix.
Battery health
Batteries lose capacity gradually. If a laptop that used to last all day now barely makes it through a coffee break, or shuts down at what looks like 20 percent, it’s worth checking before it strands you.