Every Windows computer I have owned has slowed down over time. Sometimes it takes me 2 years to notice it sometimes only 6 months. It’s often a qualitative thing that comes with a sense of everything is not as snappy and new as when I bought my computer.
The reason is that you accumulate junk and things get disorganized. A computer has enemies to its efficiency that involve; CPU, memory and disk access. Each of these components develop problems over time. My biggest peeve with a new computer is the crapware that comes loaded on it. PC manufacturers cut deals with companies to load up software and ads that only take these precious resources away from you. It’s equivalent to watching a lot of commercials instead of the program you wanted to see.
When you first get your computer de-crapify it (that’s actually the name of a good program that will help you to clean off the stuff you don’t want or need.) A big problem with all these programs is knowing which one you can remove and which ones have to stay. Should I Remove It is a program that helps even low-tech people strip off the unwanted crapware that is strangling your computer. I recommend removing as much as you can. This is good for both new computers and old. Over time you pickup programs like picking up ticks when you go walking in the woods.