Malware can often be the cause of a slow-running computer. Download, install and run the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/antimalware/
(NB: When installing it, take care to deselect the pre-ticked option to take a free trial of the Premium version).
Also launch the Task Manager (via right-clicking on the toolbar at the foot of your screen) and click on the Processes tab. Then click on Show Processes from All users. (If you're asked for permission to proceed, click Yes). Look for anything that's either got a very large figure (e.g. above 10) in the CPU column or a very large figure (above 100,000) in the Mem Usage column. That might tell you what's taking up much of your computer's processing power or memory. (If you find a process you don't recognise, post again to tell us what it's called).