I visited Apps -> Running and uninstalled the apps there that I don't use. I then installed OS Monitor and realised that a huge amount of apps run in the background with 0% CPU Usage (I have 36 processes). Many of them I don't actually use very regularly. Would it improve my system performance to uninstall them or does 0% CPU usage mean that it wouldn't? Is there a way to stop these processes automatically starting up?
When you finish using an app, Android doesn't kill the process right away. It simply stops it running, and leaves it alive as a cached background process. This way, when it needs to start the same app again, it's already present in memory, so Android doesn't have to spend time and energy on loading the app from storage again. The effect is to make apps start faster and reduce battery use. Android will only kill these cached background processes when necessary to free up memory for apps that are actually running. There's no time limit: even if it turns out you don't use the app again for another week, it still might be present in memory.
If you want to uninstall all apps with cached background processes, you'd have to uninstall every app on the device. Every app can be cached this way, even if it never does anything in the background.