What security software are you running? Did you try with Firefox in Troubleshoot Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#search If you're running Windows Pro version, I would test in the Sandbox to confirm. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/

Eset.

Disable it, reboot, and try again. I had the same issue in the past on Windows with Kaspersky. I forgot the setting that I had to tweak to resolve, but you should check their forum for help.

[https://www.google.com/search?q=Eset+keyboard+lag&client=firefox-b-1-d&sca_esv=bd24083d1d1ff32e&ei=79CwZ8bJF4qIptQPtZaGyA4&ved=0ahUKEwiG_eKAnMaLAxUKhIkEHTWLAekQ4dUDCBA&uact=5&oq=Eset+keyboard+lag&gs_lp=Egxnd3Mtd2l6LXNlcnAiEUVzZXQga2V5Ym9hcmQgbGFnMgcQIRigARgKSJUsUIUHWIMncAF4AJABAJgBhgGgAdkIqgEDNy41uAEDyAEA-AEBmAINoAL_CMICDhAAGIAEGLADGIYDGIoFwgIIEAAYsAMY7wXCAgsQABiABBiwAxiiBMICCxAAGLADGKIEGIkFwgILEAAYgAQYkQIYigXCAgUQABiABMICBxAAGIAEGArCAgYQABgWGB7CAgsQABiABBiGAxiKBcICCBAAGIAEGKIEwgIFEAAY7wXCAgUQIRigAcICBRAhGKsCmAMAiAYBkAYIkgcDOC41oAeSNw&sclient=gws-wiz-serp#cobssid=s Eset keyboard lag]