Run an extended system diagnostic - particularly on the hard drive and RAM.

F12 at powerup.

Does the system correctly identify its AC adapter (F2 at powerup to check) or does it show "unknown"?

My Latitude E6420 was running slow.

Tried every possible trick, but not much improvement. Diagnostics of  hardware showed no faults except the charger.

Finally bought a new off-brand charger, discarding the original Dell charger.

Laptop is running fast again!

The processor speed was being reduced by Intel because the Dell Charger was not working properly. With the off-brand replacement charger, processor speed is 3 times higher.

Hope this solution helps others. 

Try these steps first:

First thing is to do a “Disk Cleanup”

Check everything except “Delivery Optimization Files”

Click on “OK” then “Delete Files”

Next do a “Disk Defragmentation and Optimize”

Click on “Optimize”

If an SSD is installed you do not need to run “Defrag”

Next go to “Advanced System Settings”

In the “Performance” box click on “Settings”

Click on “Adjust for Best Performance”

Then click on “Apply”

Next click on the “Advanced” tab.

Make sure “Programs” is selected in the “Processor Scheduling” box.

In the “Virtual Memory” box click on “Change”

Click on “Custom Size”

Set “Initial Size (MB)” larger than the “Recommended:” size.

Then set the “Maximum Size (MB)” larger than the “Initial Size (MB)”

Click on “SET” and then “OK”

REBOOT