Delays or skipping for single key on keyboard
It sounds like you have a defective keyboard. Yah it's difficult, nigh impossible, to diagnose hardware via messages on the internet but that is my guess.

It may be possible to find a used, working, keyboard on eBay or the like, if you feel up to replacing it yourself. iFixIt likely has a take apart guide that would show you how difficult it would be to do it.

If you are anywhere near an Apple Store I would make a full backup of your system. Create a new admin user for Apple to use and take it in to an Apple Store for diagnosis and/or repair.