A laptop rarely becomes slow overnight. It accumulates. Every application you install adds a little startup work, a background updater, and a handful of files that never get cleaned up. Three years later the machine spends its first two minutes of life doing work you never asked for.
The second force is storage. Modern systems use free disk space as scratch space, so a drive that is more than about 90% full makes everything feel sluggish, including tasks that have nothing to do with files.
The third is heat. When a processor gets too warm it deliberately slows down to protect itself. A dusty vent can therefore cost you half your speed without a single software problem existing.