I'm getting a message when starting Nexus Mod Manager that plugins.txt is read-only and can't be managed by Nexus Mod Manager. I have no clue what this means. But lately my game has been experiencing extremely short (1-2 second) FPS drops. And huge drops, sometimes as low as 19-20 FPS, from 60. Again, it only lasts a second or two but it's getting rather annoying.

Right!

So you'll have to head over to your C: Users/whateveryouraccountnameis/AppData/Local/Fallout4

and you'll have to right click on the plugins.txt file, and under the last section called "attributes" you'll want to uncheck the Read-only button. Apply your changes and close the properties panel after that.

Also, if you cannot see your AppData folder (it's hidden by default) you'll need to change your view settings to see hidden folders.http://windows.microsoft.com/en-ca/windows/show-hidden-files#show-hidden-files=windows-7

I can't find the file/folder you speak of. When I go to Users there's nothing under there that takes me to a Fallout 4 folder.

Found it but not seeing "attributes." Only "properties."

Yes. The properties dialog. When you open that, at the very bottom there is a couple of checkboxes. Uncheck the "read only" box.