Why did you wait so long? Since you're way beyond the 30-day return period, that's not going to happen. And if you're beyond 1-year from Dell's invoice date, not delivery date, your warranty has expired. Which CPU do you have, and how much memory (RAM)? Are you running Dell Update, Dell SupportAssist, etc in the background? If they're listed on the Startup tab in Task Manager, disable them and reboot. Run the PC for a while to see if that helps. Have you scanned thoroughly for malware with something like Malwarebytes (free)? Could also be a Power plan settings issue in Win 10, but first see if disabling those apps in Task Manager helps or not. Otherwise, you won't know what is causing the problem.
Inspiron 5490 AIO with a Dell wireless mouse and keyboard. Windows 10. 1-year-old. From the very beginning occasionally the keyboard and mouse start to lag. Starts with a very slow response and ends up with no response at all. The only cure is to either reboot or sticking the USB plug into another port. After a while (maybe days, weeks) the same thing happens. I - ran Dell diagnostics- updated all drivers- removed the USB controller driver and reinstalled it- received a new keyboard and mouse from Dell. I don't know what to do. How can proof to tech service that I have the error? How can I return it as I can't show what's not working as 99% of the time it works.   
Occasional keyboard and mouse lag