I know this is a shot in the dark, but what antivirus are you using? For awhile, I had Norton - I know, don't yell at me, it's gone now - and it liked to make sure the system ran how it wanted to run. Randomly fps and performance would drop to the same lows you are getting and it would clear up after a day or so. The last time it happened it persisted for a week and I uninstalled Norton. I instantly got my performance back. Otherwise, make sure nothing else is running in the background, especially any programs that are updating without you knowing it. That's all I have to say.
im about to just install a VM
run linux on it and see how well MC will run on it
just to see if it is windows being windows.
i really hate the part of the internet that claim "linux is so much easier to use than windows"
installing software on linux is like the most difficult thing I have ever done...it is overly complicated for no reason
but yeah i got small improvements with ubunntu
just to frustrated with it to continue experimenting on it