RE: Dell Inspiron 5000 series extremely slow
"Run Diagnostic tool provided by Dell, revealed no hardware fault "
Please list exact Dell model number. Example: Dell Inspiron 5578. If you don't know, type msinfo32 onto the [Cortana] search box, and look for System Model.
In the meantime, after quick tests finished in diagnostic, you click on hard drive and then select Thorough Test Mode, also Memory as well? If no, please follow steps below:
1. Power off system. Remove all peripherals ( except AC adapter)
2. Power on system, quickly start tapping F12 continuously until you see Preparing one time boot menu in the top right corner of the screen. In the boot menu, stroll down to Diagnostic and then click on it. After quick tests, click on Hard drive, and select Thorough Test Mode. Run Memory as well. Report back any error code.