Your Android mobile is slow. It is probably a basic range mobile, or mid-range. You didn't spend a lot of money on it. Or it may even be a high-end mobile. Be that as it may, your mobile is slow and we don't really know why. These are the 3 aspects that you must understand to solve it and what you must take into account.
1.- The problem is the internal memory
Although your mobile could have a specific software problem, it is most likely that if at some point your mobile worked well and now it is slow, it is due to the internal memory. You have occupied it almost to the maximum, and the mobile can not occupy like that. You must have a relatively high percentage of internal memory free for the mobile to function well. So, you have to free up internal memory. How? Delete photos and videos that you may have occupying the internal memory, uninstall applications ... it is most likely that you yourself will perceive how the mobile is working better as you uninstall apps. Ideally, uninstall all of them and install only those that you are really going to use.
2.- Use the microSD memory
Does your mobile have the possibility of installing a microSD memory card? Use it. This will free up internal memory. Photos and videos must always go to this card. There are apps that can save your data on the memory card, like Spotify, for example. You will find these options in Settings> Applications, and in each of the apps. Use the microSD memory for everything possible, and in this way you will free up the internal memory, which should be left only for the essential apps that have to be installed in the memory of the smartphone.
3.- Restore the mobile completely
If you don't want to complicate things, something that will help and solve problems very quickly is to restore the memory of the mobile completely. The mobile will be as when you bought it, and you will lose all data. It will work perfectly again. If you have a backup of the data you need, it is a good option. You just have to go to Settings> Backup and restore and look for the option to Restore or Erase everything.