Welcome to the level 2 of computer and programming guide. Remember all from level 1 guide for programmers?
The answer to the question from the title could be that you have RAM issues. RAM is the main memory of your computer or phone and it seems that your apps are eating to much RAM. Your whole operating system (Android, iOS, Windows) is slow because it requires more RAM but how it’s possible when you uninstalled some apps but nothing much is changed? It’s still slow.
The whole idea is behind automatic updates and software upgrades who are almost always turned on. And you are getting newer versions of apps all the time. After some time those new versions become more greedy on your RAM and you start to feel like you need a new phone. Bottom line is that the companies need you to buy a new phone every 1-2 years.
The best thing you can do is to turn off auto updates for all apps who are working good immediately after you buy a new phone.
Another common issue regarding your slow computer or phone is the permanent memory, and that’s hard disk usually. Programmers are storing data directly in files and databases which use files to store data anyway. Usually, files are stored in permanent memory which is hard disk/drive. There are a lot of them but SSD is becoming omnipresent because chip based permanent memory is the most compact. Phones are using chip permanent memory because of its small dimensions.
After some time apps and operating system fragment permanent memory. This means apps and OS are making new files all the time but not deleting old ones and slowly you run out of permanent memory. Even if there is additional storage like SD card or additional disk, primary partition gets clogged and it’s not easy to remove some files manually because you can’t know if it’s important or not.
Finally, new technology and apps can use too much processing resources, which is CPU. So after some time old processor wouldn’t be able to handle new software demands.
But you are now ready for level 3 which will be more on CPU, the brain of a computer system. Stay tuned.
Remember: RAM is the main memory that's computer system. In terms of development you will have to use memory in programs and apps. You will also persist information on hard disk so it's available to your program after computer or phone is restarted. RAM is fast but not permanent. CPU is central processing unit.