Rebooting your system is an obvious first step. It is the best way to remove temporary problems. Furthermore, try vacuuming any dust out of your system. It can result in overheating which can slow everything down, as this impedes airflow. Then you should check for malware. Run a scan on your antivirus (or get antivirus if you don’t – this is a must). Then defragment your disk and run a disk clean up.
You may be hitting your ram (short term memory) hard, so it’s good to check this. When your computer is slow look at your task manger to see how much memory you are using. You can also check the specs of your computer in control panel. Have a look and maybe get something with more ram.
If that doesn’t help, seek professional advice