Depending on what you are referencing, the easiest answer is that as hardware gets better/faster/cheaper, software is written to work on what is current so newer software will run up against limits on older hardware.

Another answer is that you might have some vulnerabilities that is allowing malware to get onto your system and suddenly your system is so busy dealing with those demands and there are fewer resources available to do what you normally do.

If you maintain additional files to the registry on a system that is not unlimited in space, it will start to slow down, and ultimately stop working. You can think of it as info overload, without a registry cleaner, the computer will crash and freeze. As soon as you become aware of your computer taking longer than usual in performing responsibilities like boot up, shut down, and loading, then your registry needs to be cleaned.