The keyboard response is normal in the Windows login screen, but is very slow while inside Windows. The problem occures at all times, in all programs.
The problem is not sticky keys or filter keys.
My only paranoid idea might be that a malicious keylogger is running. If it has a hook which takes a non-trivial amount of time to do processing(such as logging to a file or even sending across the network), then it will create a delay in keystroke processing. I would suggest running a scan with something like Malware Bytes: www.malwarebytes.org
Edit: Consider booting into safe mode to see if the issue is still present. If it goes away in safe mode then you know it is probably something running at startup, such as a windows service(a background program), or driver.
If so, use Run->msconfig and disable one item under SErvice or Startup tabs, reboot, test keyboard, disable a different service, reboot, test, etc. Using the process of elimination you hopefully can determine the problem service or startup program, if that is the issue.