Use the Performance Troubleshooter
Open the Control Panel and search for troubleshooting. Run the troubleshooters under System and Security, and it may find the root cause of your slowdown. For good measure, run through the other troubleshooters, including Search and Indexing, Hardware and Devices, and Windows Store Apps. Also go to the old-style Control Panel’s System and Security > Security and Maintenance page, click on Maintenance, and hit Start Maintenance. This happens automatically on a schedule, but if you’re experiencing slowdowns, it’s worth trying. If your PC is still hopelessly bogged down, you can use the Windows Security app’s Fresh start option, but be warned that doing so removes your installed programs.