Computers rarely get slower on their own. What changes is the amount of work happening in the background: more programs launching at start-up, more browser tabs held in memory, more sync and update tasks running quietly. The hardware is doing the same thing it always did, but sharing it with far more work.
Memory pressure is the most noticeable form of this. When the system runs out of physical memory it starts moving data to storage, which is far slower. The symptom is characteristic: switching between apps becomes sluggish even though each app works fine on its own.
Storage pressure has a similar effect. Systems constantly write temporary files, and when the drive is nearly full there is nowhere efficient to put them. Keeping ten to fifteen percent of the drive free avoids a whole class of mysterious slowdowns.
Heat is the third factor. When internal temperatures rise, laptops deliberately reduce their speed to protect themselves. A machine that is fast for ten minutes and then slows down, with the fan running loudly, is usually thermally limited rather than short of resources.